I just looked into this. I was able to go to 250x250. 300x300 still worked, but what happened for me was the grid disappeared.
So basically it looks like there is a maximum amount of memory/video memory that your computer has. It seems like (on Chrome at least) when you reach the maximum of this memory, the Canvas layer can crash, which is what causes the grid/fog of war to disappear. The other stuff (maps, tokens, etc.) seemed to work fine for me.
So unfortunately while there's no limit baked into Roll20, it seems like there's going to be some sort of upper limit on how large a map can be based on your individual computer. Also keep in mind that this means there will be an upper limit for your player's computers as well.