45mm is a great general-use size and it is the size I recommend to rotary cutter newbies.
Clover produce some of the best quality craft tools available. I've tried the 'other' brand of rotary cutter and in my opinion the Clover cutters are simply nicer. Clover cutters are: more comfortable to use, better finished and it doesn't hurt that they look prettier. The handle has a comfy soft grippy cushion and the blade cover leaves just enough of the blade exposed to cut the fabric - whether it is held upright or at an angle. For right or left-hand use.
If you haven't used a rotary cutter before you are in for a treat. Rotary cutters greatly increase the speed and accuracy of fabric cutting. You can cut multiple layers of fabric/interfacing/interlining with one sweep of the razor sharp blade. A toolkit essential for all bag/quilt/clothing makers. FYI my own cutter is the 60mm size.