[trebonus]

I have a hot water heating system with radiators, and I want to replace the radiators in the upstairs bathroom and bedrooms (2) with hot water baseboards. Can radiators and baseboards be mixed and if so, how do I determine the size of the baseboards?

[Airwren]

The quick answer is yes you can change them. The way that is mostly followed is approximately the length of the window, although that is a haphassard approach. The way that should be done is an accurate heat loss, followed by the output of the baseboard per foot.

[saffle]

I have a hot water heating system with radiators, and I want to replace the radiators in the upstairs bathroom and bedrooms (2) with hot water baseboards. Can radiators and baseboards be mixed and if so, how do I determine the size of the baseboards?

Just be aware that baseboard units have much less heat output. I've elected to go the other way. I salvaged old cast iron radiators and installed a few instead of the baseboards that went along the whole room and did not heat sufficiently. We are now warm and toasty.
You would be wise to get a pro to evaluate what you want to do.