Sorry as you may have guessed I'm having a bit of a nightmare with Louis, currently feeding him some fruit in highchair so may have 2 mins if lucky!!
The minkis are generously sized. the slinki is slightly smaller in fet and the velro doen't cross over at the front making it a big slimmer too. As pp said stuffing with hemp will give best absorbancy for slimmest fit. If you got a medium it may not fit her at all and if it did you would need to buy the large anyway soon. Its will seem a bit big but as its night it doesn't really matter for clothes etc. If the large fits from 18 and she is nearly 22 I wouldn't think the large will be too big at all.
If you are making your bobosters yourself a metre of hemp (Ã‚Â£8 ) will do around 8 boosters so works out really cheap, yes a bit more than terry but soooo much better. I just made mine with hemp and then fold a mf cloth in thirds(89p for 3 at tescos) over the top before inserting to give the quick absorbtion. for night 6 layers of hemp and the3 thin mf layers is really thin, about 1.5cm I'd say and that does Louis the night and he is quite a heavy wetter. If I only have 4 layers of hemp then I need the mf cloth plus 3 layers of terry which gives a much bulkier nappy. Hope this gives you some idea of the diff in absorbancy. for the extra coupel of quid I'd go hemp! I had a peice of hemp left that I still haven't got around to actually sewing and it hasn't frayed (been washing since xmas), its quite big and folds nicely into 6 layers to go in a night nappy, the only thing that has ahppened is the edges have rolled a bit. Terry does fray. My wipes are all just fleece and they can be cut and go no sewing.