Uploading code size

There shouldn't be an upload limit on normal uploads, no... although if you're uploading with 'Save on send' then there currently is a limit as you have to have enough RAM to hold the data (it can be fixed in an IDE update now 1v96 is out though).

Is it possible that the code you're uploading is actually doing some calculations/work as it is being uploaded?

If so, Espruino could potentially be executing it so slowly that the input buffers get full. Usually if you stick the code you want to run in an onInit function then that'll sort it though.

If you run E.getErrorFlags() then it might give you something like a FIFO_FULL error that'd tell you if it's happening.