Today I installed both FFH2 0.41g and FF 0.51c. In addition to FFH2 I installed Flavormod 2.7.6 for FFH and now I can pick giant maps.

Not so for Fall Further, where Flavormod should be included.
Any ideas why this happens?
Which version of flavormod is included into FF?

Thank you:)

well, it's not that they're not working, they're just not there. Most likely, that feature wasn't there to begin with and FF hasn't added it yet.

I do wonder how a map even bigger than huge would even be playable. I tend to get MAF errors and unbearable slowdown on huge maps, even.

Jean Elcard decided not to merge that particular feature into FF.

Mostly because FF maps that size would run incredibly slow. ;)

Thank you

Why does it then work for FFH2. Is FF more prone to MAF than FFH2?

We have more stuff, is the simple answer. Xienwolf has his eye on a few tricks that could make a massive difference to performance though, so we'll see in future.