iso booth size question

[genericperson]

hello,

i wanted to post this in the high-end site, so people who have an experienced perspective could chime in hopefully

i'm building an iso booth, primarily for vocals. has anybody noticed a minimum size, that if you go smaller, people think it's too small?

the one i'm kind of in the middle of building is definitely bigger than this:
http://www.vocalbooth.com/products/silverseries.html

but i'm still concerned about size.

also, i'd like to do it without glass, and just run an lcd screen loopback for visual communication needs. on the other hand, building one of the walls with two sliding glass doors to form a cheezy soundlock could be cool. so any thoughts on "glass" would be welcome too.

thanks for any insights