It's a timeout, not a size thing, so the biggest image you can upload depends on your connection speed.
If you have a big file and fast connection, it will upload.
If you have a medium size file and slow connection, it will time out.
I have a relatively slow upload connection here, so I don't upload more than three images at a time or I get a time out.
As all big images will be downsampled, you may as well crop them yourself.