I am looking to upload a full color model but was not sure if overlapping UVs are allowed, and if there is a size limit for the jpegs?
As long as your zip is below the size limit you should (probably will eat my words) be ok. The resolution of the prints isn't super, so you don't need a huge image file. The best way to find out is to upload your zip and see how the render looks. From there, if you have any issues we can help you sort them out.
We don't support the use of multi textures, where one face is colored by more than one UV map. As long as each face is always covered by one and only one UV map, you're free to use them however you like! So yeah, mirroring them shouldn't be a problem at all.