Some things can only have certain sizes ("inflexible size"), for example Lego Duplo are a multiple of about 16 mm (I think). Other things are "flexible in size", as they don't have a restriction, or much less restrictions.
Maybe flexible isn't the right / best word either?
My use case is a bit technical, it's a computer science problem. The "inflexible size" applies to certain things (hash tables) that can only be of a size 2^n, for example 2, 4, 8, 16, 32, 64,..., 1024,... The "flexible size" ones can have any size that is a whole number, that is, 1, 2, 3, 4,... 1000,...
Maybe "flexible sized" would work, but I hope there is shorter word.
Sample usage: there is something called the "Cuckoo Filter" which is inflexible in size, that is, can only have size 2, 4, 8,... "The Cuckoo Filter is a probabilistic data structure that supports fast set membership testing." Now I need a use a different term for "Cuckoo Filter with flexible size". But "Flexible Cuckoo Filter" doesn't say it's about size, and "Flexible Sized Cuckoo Filter" is quite long.