Sounds strange I know but I'll explain. We live in a chalet style house with the bathroom downstairs and now my twin DDs are dry at night I'm trying to find something like a potty to leave in their rooms during the night in case of a wee because they are a bit nervous about going downstairs in the night on their own.
Normal potties are no good now because my girls are quite big for their age and the last one split mid-wee
My ds is small for his age (5 but I've just had to buy him trousers for a 3 yo)and we have one of those Mothercare potties. It has been too small for him for quite some time so it may not do for your girls.
Sorry I haven't got any good ideas but I will watch this thread with interest as we are hoping to 'potty train' ds some time soon and need a big potty ourselves. (he has sn btw, not just lazy!)
I use this one for my ds. Just to give you some sense of scale - he is 2.8 and weighs 18 Kilos (40 pounds) he comfortably uses it. It isn't pretty but hey who cares as long as you don't get wee everywhere.
Sandyballs - my dd is 4yrs 2 months and is about average height/weight. We have one of the 'potty chairs' that look like a teddy bear - sorry not a very good description but I hope you know what I mean! They come in all sorts of bright colours and are available in all general baby shops/market stalls etc. I think size wise it would be fine for your dds, although don't know about how long they last (only cost about £6.99 though, so doesn't really matter).