room size for low power amps and efficient speakers

I'm designing a dedicated listening room and would appreciate input on the size given my amps and speakers. My tentative plans call for splayed walls and a sloping ceiling. Front wall 3m (9.84 ft) back wall 4m (13.12 ft) side walls 6m (19.7 ft) ceiling 3.5m sloping to 3 meters. One tube amps is 25WPC the other is 18WPC. Speakers are 94 - 97 db sensitivity. I do not listen at high sound pressure levels. Too big, too small or just right?
Bigger is always better regardless of speakers.

Non-parallel is one of those things that seems like a really good idea until you dig into it. Seems good because it creates more modes that are low in level which is better than the fewer/bigger modes parallel walls create. But the angles tend to focus the sound and so still need to be treated. $$ Non-parallel is a lot more expensive to build. It can be done right, and better, but not based on tips from random interweb wanna-bees. You need to pay for professional design, which adds even more to the cost. Which is fine if you're one of these guys who is made of money. Normal guys with limited funds will find they can get a lot more out of that same money by budgeting a lot less for the fancy architecture and a lot more for really good wire.