Just a case of trial and error. Try every room in the house and listen to what sounds best to your ears. Also try various positions of the cello in one room and move the mic around! If the room isn't dry enough to your taste, bring in some blankets, curtains and carpets.
Recording with headphones would make it easier because of the bleedthrough you would get if you use speakers.
Good luck and have fun!
Gomp

I can only really tell you what i generally do after some trial and error: I tend to record cello with an LDC in the lounge, it has a tiled floor with a rug partially covering the floor (without that there seems to be too much low end), the room's about 18ft x 12ft and i place the cellist just off centre of the room, facing towards a corner (they're about ten feet away from the corner). I position the mic about half a foot above the bowing, around a foot and a half from the instrument. This obviously set in stone as there are a lot of variables, such as player and instrument, room temp due to the nature of the instrument etc etc, but it's my starting position.

I've found the reflections in bathrooms on cello are too quick, i like that sound with picked acoustic guitars for instance, but i don't find it appealing for cello, might be to your taste though, who knows?

Hope that's of some use.