Speaking for the three major leagues, currently MLB has a handful of players taking 42s and a few more taking 44s, but I would agree that 48 is a pretty normal size these days.
Of course, sizing was figured differently in the "good old days". In the 1960s, players such as Hank Aaron, Ernie Banks and Billy Williams were often outfitted in size 40 flannels. One of Don Drysdale's last jerseys (1969 Dodgers) was a 46, which today is considered slightly diminutive by MLB standards.

Warrick Dunn at a 38 is the smallest that I can remember offhand in the current NFL. The tailoring done to make the jerseys skin tight makes comparisons more difficult, as the era of Jim Brown, Gale Sayers and Dick Butkus didn't feature lineman's cuts, skill position cuts, and the like on a regular basis like we have today.
Many older (1950s/60s) NBA jerseys were 38s and 40s, with untagged extra length. These days, I have seen a few 44s (Allan Iverson comes to mind, and he took a 42 pre-Reebok). Today's NBA players, for the most part, like loose fitting unies, unlike throwback John Stockton, who wore 40s and 42s during his illustrious career.
It's my own arbitrary figure, but I consider the late 1980s/1990 to be the turning point for sizes ballooning up.
Dave M.
Chicago area
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