1st off the rim choice is pretty simple..any 4/110 Honda standard bolt pattern rims will fit. 8" diameter is stock for the SX, but a 9" or 10" would do if that is the tire diameter you find.
The stock tires are 22x11x8, so you could use that as a safe starter size..22 or 23" diameter on the SX is a good area to be in..
Now the other issue you will discover is a trikes design makes them very unfavourable in the snow because the front tire will tend to 'plow in' instead of floating over the snow..
Our good friend Scootertrash is a member who has been playing around with that solution..he's fabbing a front ski using a snowmobile ski and making a mounting bracket whereby you remove the front wheel assy and install the ski kit!!
One way or another I strongly suggest that a front ski is needed in addition to a pair of good snow tires for the rear!!
Maybe even something like this..
http://www.ebay.com/itm/ATV-Tires-to...0f0515&vxp=mtr
Current toys..
1986 Honda 350X..trail bomb!
1985 Honda 250SX..my main mudder
1985 Honda 250ES..Back in Black Trike
Current non-trike toys:
1990 Honda TRX300FW
1995 Seadoo GTX
1998 Polaris Indy Lite 340(Nearly new looking)
1998 Polaris Touring 500
1998 Club Car (electric)