Welcome to the forum Joe, I would estimate that you would need an XL size T shirt for a 45 inch chest, of course it all depends on how you like to wear your T's of course. Maybe a Large if you want a slimmer fit and up to XXL for a loose baggy fit. The T shirts are very well made, particularly the embroidered designs which are pretty heavy too. Naturally paying $100 plus for a T shirt is considered expensive by most people but also consider that they are made in limited numbers too and this increases the overall cost. Anyway best of luck buddy look forward to seeing your purchases