Excellent question that often comes to mind, especially when you're talking about Luxury boutique hotels. It's very important to stress that how the bedroom is arranged is also of crucial importance. I stayed at the Munchen Palace in a single room over the week-end. Size was 18 sqm but layout was well thought out with a lot of storage space in the walls which made the room look more spacious and overall it was more than enough.

(For some pictures, you can check my review on TripAdvisor): http://www.tripadvisor.fr/ShowUserRe...ECK_RATES_CONT

But I was alone and the room was cute.

Normally I wouldn't go for something below 25sqm and just like you with 30 sqm, I'm starting to be happy, especially when I'm staying with my girlfriend. 35-40 sqm is an ideal size. Over this, it's more extra space but it's not really needed I would say.