In the books
The books don’t give much description of the size of the map. This is how it’s described when Harry first sees it:
Fred pulled something from inside his cloak with a flourish and laid it on one of the desks. It was a large, square, very worn piece of parchment with nothing written on it. Harry, suspecting one of Fred and George’s jokes, stared at it.
— Prisoner of Azkaban, chapter 10 (The Marauder’s Map)
The book seems to imagine it as a single plane: I can’t find any mention of flaps or folds like in the films, there’s talk of Harry “rolling up” the parchment (imagine what that would do if it was a folding page), and everything seems visible at once.
We don’t know how large a desk is, and how well it fits on said desk, but it gives a rough idea of how big it might be.
In the films
This map is much more intricate, with folds and flaps and fiddly bits. I suspect this is more realistic a design for mapping most of Hogwarts Castle.
The Noble Collection will sell you a replica map based on the prop, and their product page gives us the dimensions:
A detailed full size Marauder's Map replica printed on parchment paper. Measures 15.5 × 72 inches open, folds to 15.5 × 8.25 inches.
When they say “open”, this means the largest folds: there’s additional area because it doesn’t count some of the smaller flaps, such as right at the centre of the map itself:
That seems like a more realistic size, if you suppose that the different floors appear in a horizontal row across the map.
J.K. Rowling has drawn her own map of Hogwarts, which appears on the Prisoner of Azkaban DVD extras and can be seen on the HP Lexicon. The detail is impeccable and I feel it could easily substitute for the Marauder’s Map, yet it fits on a single sheet of A4: