Wetsuits stretch as you break them in, and as they fill with water. So I’d go with the small, the first few dives it may be annoying and you may have stretch the neck out with your hands a bit while wearing it during surface intervals, but eventually you’ll be perfectly comfortable in it. The medium will allow too much water between your skin and the suit itself so while you’ll be comfortable on land, you may be colder than expected submerged. Just my $0.02.