I have a completely free partition of 19.5 Gb (Windows is installed in another partition.) When I start Wubi, it suggests that I use about 12 Gb for Ubuntu. 

The question is: Should I go with Wubi's suggestion or should I use the maximum possible that Wubi is allowing me (19 Gb)?

Also Wubi isn't letting me set a custom size (like 13.8 Gb for example). Is there a way around it?