There isn't a maximum layout size except for how big a number you can fit in a 64bit floating point number, so ~1.7*10^308.
What Fengist describes is the maximum texture size for a text object, but this is dependent on what your graphics card can handle. Basically how big of an image it can handle, but then again this has nothing to do with layout size.
Anyways how many objects you can have on the layout will vary a lot. The two things that will cause a slowdown are:
1. Amount of things on screen at once. It takes time to render stuff.
2. What objects actually do and what your events do. This is a very wide range though. It's possible to use well above 1600 though.