Are there explicit numbers for file weights and token numbers not to be exceeded?

Now, I realize this is complex, depending on many factors, like the speed and power of player's client machines and server/network buffering, but is it listed in the Roll20 documentation anywhere: what is the best file weight for images used for tokens what is the best file weight for map images how many tokens can be allowed on a map page before things irritatingly slow down? I know hard numbers are hard to come by for this type of problem. But I'd like some broad boundaries at least. Are those listed anywhere?

There are individual file limit sizes, which you can read about here . The recommended resolution for images is a maximum of 140px x 140px, because anything over than that will not yield any greater resolution on the VTT, which is set to 70px x 70px at 100% zoom. When you upload any images to Roll20, they are automatically duplicated and rescaled to 4 sizes that will automatically display according to your zoom level. Those duplications do not count towards your storage limit. When setting up a page, you'll get a warning if your page size is set beyond 100x100 grid squares. It's only a warning, and I have had several pages larger than that with no problem. The biggest issue is Dynamic Lighting. The more tokens you have that emit light and that have vision enabled will drastically slow down your machine. There's no hard number to rely on, because it will depend so much on each individual user's computer hardware, and how complex the lighting setup is. It is highly recommended to turn off vision on every NPC token for this reason. 

One small correction. The VTT will zoom up to 250%, so the greatest resolution with any meaningful difference is 70 x 2.5 = 175 pixels per grid. Roll20 maps in modules are 140px resolution when supportable, tokens typically higher, most likely for future-proofing, should the specs ever change.