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jgibson is mostly correct but there are a couple of extra things.
Equirectangular is the term used more in the context of graphics. In cartography/geography, "Equidistant Cylindrical" is the more typical term for this projection. Both do describe the same thing, although in the case of geography it can be complicated when the surface being projected isn't a perfect sphere (which is way more detail than fictional map making needs).
Cylindrical projections are, as the name implies, based on a notional "cylinder" that you project the map onto and then "unroll" into the map. In the "normal aspect" the cylinder is aligned with the axis of the globe, although there are other aspects, you don't need to worry about them just now. So assuming a normal aspect, the cylinder can wrap snug around the globe at the equator, or it can be smaller and slice through the globe, in which case it intersects the surface at two parallels (latitude lines). In math speak we call these "tangent" (just touching) and "secant" (Cutting though). The parallels where the cylinder touches/cuts the globe are called the standard parallels, and they are were the map minimizes distortion. As you move away from them, distortion increases.
So, in the tangent case (the standard parallels are both the equator) The width of the map is the length of the equator, while the height of the map is the distance between the poles (North-south distances are preserved in this projection, hence the name "equdistant", other distances are distorted.) Assuming a perfect sphere, that makes the north-south distance falf the circumference, and the east-west distance the full circumference. So the aspect ratio is 2:1 as jgibson said. This particular case (normal tangent equidistant cylindrical) is also called "plate carree".
However, you can also set the standard parallels elsewhere, and get smaller aspect ratios. Again assuming a perfect sphere, you multiply the width by the cosine of the latitude. (If you know a bit of basic trigonometry, you can probably work out why this is the case fairly easily). If you don't know any trig, just use a scientific calculator enter the latitude, then press "cos". (On most calculators with trig functions at least. Fancier ones which allow you to enter multiple operations before executing generally want the "cos" before the latitude instead.)
If we assume your map is meant to be a full globe equidistant cylindrical map, then the standard parallels work out to almost exactly 45° N/S. This results in similar pinching to the tangent case, though not quite so bad, but with stretching at the equator to balance out the reduced distortion at the poles.
I also noticed that your landforms conform a bit to well to the rectangular map to look natural. Trying to fill things in neatly is a natural inclination you need to try to overcome when making up fictional terrain. Try to avoid aligning things, particularly north-south or east-west.
Your biome placement and mountains also look a bit off to me, but I'm not so strong on geology, climatology, and ecology. You should probably try to separate the geology from the climate/biome. Mountains may affect the climate and ecosystem, but they aren't a climate or ecosystem in and of themselves. You can have a wide range of different ecosystems on mountains. We have fairly drastic variations in precipitation here in British Columbia due to the mountains. An individual mountain range usually isn't very wide, so when you have a significant area with mountains, it tends to be a sequence of parallel mountains with valleys in between. When most people who don't live in western North America think of as "The Rocky Mountains" is actually a lot of mountain ranges crammed together including the Coast, Cascade, Columbia, Insular, Olympic, Rocky, Sierra Nevada, and Sierra Madre mountains. Squashed in among them are everything from rainforests to deserts. I think there's only one place on Earth where a large contiguous area has been raised above the treeline rather than just peaks or ridges, and that's the Tibetan Plateau.
Also, mountains are associated with boundaries. No necessarily coastlines, but where they lie between two sections of flatter land, those two sections tend to be discrete "lumps" rather than one cohesive lump with a mountain range through the middle. Ranges that violate this, like the Urals, tend to be old and worn down as the boundary they were created by has fused solid. Concentrated "lumps" of mountain in isolation from any sort of linear structure are also a bit un-natural looking.
