Whether you actually have that problem or not depends on just how much screen space the models would be taking up and how large the textures are relative to that.
If the models are "typical" as in not usually taking up the whole screen, especially if they are scenery, the texture size of 1024x1024 is likely plenty, however if said model is a focal point in your game/render, you would maybe need 2048x2048. It also depends on if they are going to be rendered with normal-map and/or detail map(usually meant for terrain, but sometimes you'll find it on other things). If you need to, you may be able to re-create the texture at a bigger size yourself, using the smaller version as a reference. This won't affect the uvs at all, as long as you keep everything relative.
I'd say check the texture size, and if you need to, you may be able to contact Turbosquid and/or the creator of the model to confirm if it is fit for your purpose.
Also, on the other problem, I don't think you'll have oddness when scaling models, as long as the objects fit right. In fact, you are scaling to make things fit, so at least the geometry itself shouldn't look oddsized, because that is the point of you scaling in the first place. 3d is all relative. It doesn't matter much exactly how big a human and a car are as long as about 1 human fits into a car seat. The exceptions to this issue is physics simulation(which behaves weird when it seems huge objects move to fast or the other way around) and camera frustum issues. If everything is too small, they may be invisible because you'd have to get too close to the camera to see them, and likewise with high distance, if you have to scoot the camera very far away to see objects at the right size. Renderers(and 3d engine cameras) have a frustum that counts as visible, which tends to be like from 0.3 to 1000 units, and anything outside of that doens't get rendered.
