How large can a map be? Unfortunately, this is one of those 'how long is a piece of string' questions. It depends upon a number of factors that I'll go into in a moment - but there is a limit and when you hit it, the app will 'crash'. The most likely place you'll bump up to the limit is on iOS where memory is extremely constrained.

What factors affect the max map size?

Device type. The original iPad1 had 256MB of memory whereas the latest iPad Pro has 2GB (8x as much) and a Mac or Windows machine will have upwards of 8GB
How many (and what type) of other apps are running also.
How many topics are in the map.
How much text is contained within the map.
How many (and how big) are any attachments/images in the map.
How many topics are actually visible (not folded.)

Either way, you should have no problem with maps containing a couple of thousand 'normal' sized topics on any device.

Tips to avoid out of memory issues:

Avoid large topics. Topics with lots of text or large text or images all use lots more memory. Oval topics use more space than their rectangular counterparts.
Avoid zooming in. Zooming in to 200% will quadruple the area taken up by topics (and thus quadruple the memory required.)
Avoid large numbers of topics that are permanently visible (i.e. try to keep branches collapsed.)
Break up large maps into sub maps and use hyperlinking to knit them altogether.

NB: On iOS, if you are unable to even open a large map, then you can try the follows:

Open 'System Settings' (under the gear toolbar button)
Scroll to the bottom and set 'Collapse branches on load' to ON
Open the offending map.