A bit more about perspective lines. Supposing you want to make the rail road ties look evenly spaced or have fence posts evenly spaced or anything that repeats while receding into the distance. How to do that is explained in the black and white image. (BTW I just copied these off the internet. Its just quicker) (edit) I just realized the black and white image didn't explain the first step very well. So I edited it with colored lines. I'll tell ya whats going on. The 2 green lines are the receding lines. The 2 lines with red arrows help establish distance. Draw the first vertical line and find the center, the yellow line that goes from that center to the VP will help scale distance. Draw the next vertical line with the red arrow. This vertical line can be anywhere you choose. you're creating distance here. The rest of the "posts" will keep that distance in scale as your recede into the distance. Now you're on track, no pun intended. Like it shows in the picture, if you were to take a straight edge or ruler to connect the top of the first line to the center of the second line, and project that straight edge to the bottom receding line, that would give you the next spot for your telephone pole or whatever you're drawing. This works for 3 point perspective too, The only important thing is that the lines that represent the vertical all appear perpendicular. Take a look at the drawing with the colored steps. look at the side of the steps and on the bottom you should see reference lines that look exactly like the tutorial for the evenly spaced objects tutorial in the first photo. it's the same technique. You have to learn the first one before you can do the stairs. Mostly I wanted to show this because of the rail road ties. Keep in mind in real life nothing is this perfect. This stuff is important because it helps create a feeling of space and getting scaling proportions more "correct". Getting the proportions more "correct" gives you more "room". When painting in perspective objects get smaller when approaching the horizon but the danger is making things get too small - too soon. You'll be able to create more space by getting those rail road ties correctly spaced to begin with. just keep in mind this technical form of perspective is for scaling and creating space and not for making thing's "perfect". Well actually it's helpful when painting any man made structures but only as an aid. Following this stuff to the letter will make your paintings too rigid and the painting will suffer because of it.

Everyone gave you some wonderful advice. Keeping what they said In mind I have a few things to say. your rail road gives you a fantastic base for scale, as mentioned by others. You want to use that opportunity by adding more cacti scattered throughout the scene between the front cactus and the beginning of the mountain range. Doing this preps the viewers eye to get a sense of scale. You're drawing the mountains small but you want them to appear big. The only way to do that is in reference to something. The first cactus next to the rail road sets the average size. Both TK and Nicolas are very helpful in understanding this better. It might sound like I'm just repeating what others said and I am to the degree i'm emphasizing you should use more iterations of cacti. The farther back in "space" the more "room" or "area" you have to add lots and lots and lots of tiny (because of scaling) Cacti. The trick is to make the grouping of cacti look like it would in nature. My main take away for you here is to use lots more cacti, here is where you help the viewer get an idea of scale. But it only works if you use what Steve, Nicolas and TK. These tiny cacti are tiny, I mean They'll look like little blobs or lines. You wouldn't even know they were cacti if the foreground didn't already have objects that could easily be read as cacti. The point in space your mountain range begins you should keep in mind the base of your cacti and the base of your mountains will be very close, your cacti blobs will overlap your mountains, but the shape of your mountains will be much larger then your cacti blob, just the base will be almost lined up, not the tops. Like Steve said objects like the different species of trees and rock formations will come in different sizes. So when you make iterations of the same type of cacti people intuitively conclude that all those cacti are going to be roughly the same height. If you feel ambitious you could include different species of cacti to help with scaling, but just make sure you have many iterations at different placements in perspective. Using Steves extremely helpful drawing as a reference. . . He has large rocks in there, take note of how they exist in relation to the cacti. The same species of cactus will generally be the same size but rocks are unpredictable in this way. I'm not saying you cant use rocks, it might be handy to have a large rock next to a cactus which could make it the mountains seem even bigger, but the key here is in how the cactus, the large rock, and the mountains overlap in space....see what I mean? If one of your pictures uses something that doesn't scale well, like rock formations for example, then you would be relying on texture. Referencing back to what he said about atmospheric perspective.

Hello, so size of stuff is pretty easy to tackle. You have to place the horizon line and define at wich heigth it is. I could write a tons around that but there is a wonderfull little explanation made by Kim Jung Gi just here : https://youtu.be/CGLIQvNk3zY. You can also find a method in the book “Framed Perspective” by Marco Mateu-Mestre. To be honest I’ve looked at both and regarding proportions and putting stuff to the right size I have found Kim Jung Gi’s explanations clearer. Hope this helps ;)

Hey Crystal, @TK advise is correct, It's all about scale. It can be a little different with trees and plants because they can grow to different sizes, but generally that's how you figure it out. The other thing is when you add color, you must use atmospheric perspective to help sell the distance. You do that by lightening and desaturating the colors. I did a quick sketch and painting to show you what I mean, hope it helps :)

You can keep the scaling consistent by dividing your figures into units and then projecting the measurement wherever you want in the picture. Here's a basic example using a single vanishing point