is 25kw or 30kw combi boiler suitable for 6 room house?

I'd personally always advise going high with a combi, the higher the rating in general, the better the hot water performance.
The heating can (in a lot of cases) be adjusted down to suit the system.
The difference in HW output can be 2-3 litres p/m higher in a larger rated boiler.

Personally, I have a Vaillant 837 in a three bed semi. 15+ Litres of HW and the CH is rated down to 14kw to suit the system.

Carefree
August 2010