I am working on a map which is really ambitious. (it is gonna be heavily detailed and HUGE)

So how big of a map can i make?

That's quite a vague question actually, and there isn't really a direct answer for that. However, it mainly depends on how sloppy or organized of a mapper you are. It all comes down to a few things:

Engine Limitations: (probably the most annoying part about a large map)
This could include models (model spam - a huge misconcption of what detail is), effects, brushes; literally anything that is used in the map I would strongly suggest that you keep in mind the amount of entities you use in your map by pressing 'M' in radiant.   You can compare these values to the ones found here:
http://wiki.modsrepository.com/index.php?title=Call_of_Duty_5:_Engine_Limitation

One main issue will be pathnodes, and finding the best spacing to optimize your map and not hit the limit!!

Sloppy Mapping:
This involves poor mapping habits that a lot of people do. Not all sides of a brush should be textured, poor light mapping can cause a lot of errors in a large map (usually from stretched patches, and cylinders), brushes colliding with each other, lack of or misuse of portals (portals may seem like a good idea, but not if there is nothing really to portal), and any small minor errors which add up in larger maps (can be ignored in smaller maps).

Light Grid Limitations:
From experience with Frostbite, I have found that there size of a light grid can easily cause errors in a large map. Take advantage of the fact that a map can have up to 31 light grids, and that they should only be used in areas which the player can access. It is best to use more light grids on a smaller scale, which allows the engine to calculate less and more controlled areas.

Keep in mind that there is no, distinct limit for a map size, but the more that you have in your map, the more difficult it is to run. Many people won't be able to run your map at all if there are even a few bugs throughout the map, due to scale.
I hope this helps!