Yes, early baseman mitts are probably the most ignored of the position mitts and yes, probably more scarce. The catchers mitt and baseman mitt were one position mitts but there were usually more than one catcher and several catchers mitts available to a team. First base maybe not quite that so.
As to sizing, the fit of the hand inside will tell part of the tale, width another as one poster wrote. Overall size yes. I was very lucky enough one day to put my hand in a gamer Lou Gehrig Ken Wel mittt and I marveled at the width of the mitt, just very, very wide.
Also correct is that the basemitts grew in size as time went along until we got into the oblong oven mitt styles of the late 1920s 1930s.
Popularity of the basemitt increased dramatically with the introduction of the Rawlings Trapper Claw. Derived from a heavy webbing mitt put together by Hank Greenberg (Remember the key to the basemitt is catching the ball and not necessarily a quick release- the snagging and holding of the thrown ball is critical. Hence Greenberg figured.
The baseman mitt or glove might have been the second position mitt/glove utilized as this player had to take more throws than any other spot. I believe, in the Spalding story, when A. G. Spalding donned one of the first mitts (accepted) in the field, he was at first base.
Just some thoughts.
