Nymphs should be kept in a smallish container until they are a bit bigger so they don't have trouble finding prey. (Unless you want to hand-feed for awhile) I really don't think there is such a thing as 'too big' of an enclosure- so long as you can find them when you need to, and food is readily available. Some people keep their mantids in large vivariums but take their mantid out for feeding. I think you might've forgotten to share the picture because it isn't appearing for me! I know this post is old but a few weeks ago I was wondering about the same thing (is there such a thing as 'too big' for mantids when it comes to enclosures?)