If you are ever shopping for a Masonry trowel, and you want the very best, W. Rose is the best.
http://www.wrose.com/Products/BladePatterns There is a number for each size. I would suggest not going over #10. Bigger longer trowels may seem like you could get work done faster, but unless you are well accomplished in using a trowel - you'll waste a lot of mortar and your work will be sloppy.
If you're looking for a masonry trowel to garden with - a cheap box store one will be fine.