Not sure about your road/off-road and flat-bar/drop-bar preferences, but the Jamis Renegade and Norco Search XR are the two off-the-shelf gravel bikes that come in tiny sizes (that aren’t horrible compromises) that I know of. They’re both designed around 650b x 40ish. The Norco specs state that it can also fit 26x2.1".
And both sell framesets.