Hello and Welcome,
to a great tool that not only will reward you, but frustrate you as well. Just like it does myself.
First, SEARCH! You may get many more responses if they know that you actually put the effort in to find the answer yourself.
There have been times where I searched the forums for hours trying to find the answer to my question or problem. If you still can't find it, inform the readers of your post that you actually did try finding it. Mention the posts that may be similar to your question, i.e: "this post talks about max sprite size, but..."
Second, when you do search, change the wording of your question. After you try "what's is the biggest layout size allowed." Try "Maximum layout size."
You may get many of the same posts, but there could be some that are different.
Now, to the question at hand, I believe there really isn't a MAX for layout size, which, again I believe, would be your Tiled Backgroundlayer. On the next layer(s) would be other things like Mountains(on layer 1) palm trees(layer 2) and then the layers on top of those would be your sprites, i.e: player, enemies, etc. etc.
I did find this post in a matter of a minute. Maximum Background
Hopes this helps.