I'm sorry guest, but I have no experience with P. 'Little Prince', I can find only one reference to it on the web, and that has no mention of its ultimate dimensions. Your best bet is to find it at a garden center, and read the tag or ask the local nurseryman. The single mention is in an obsolete list from Tendercare Mature & Formal Hardy Plants:

http://www.tendercare.co.uk/

so you might also try to contact them for more info.