身材 refers to overall body shape and proportions. 身材好 is a common compliment meaning someone has a nice figure. It's considered less sensitive than directly commenting on weight.
mistakes
Don't confuse 身材 (figure/build) with 身高 (shēngāo, height) or 身体 (shēntǐ, body/health). 身材 is specifically about shape and proportions.
