This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Up to v1.86 of the software the upper limit was 16 MB of memory. As of v1.87 that has been raised up to 32 MB of memory. However, for some odd reason, after 16 MB of memory the memory can be only configured in 4 MB chunks. Consequently the possible values go 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,20,24,28 & 32 MB.