Reach and stack figures are somewhat useful when comparing similar bikes, i.e. road bike A versus road bike B, but less useful when going from a road bike to a flat bar bike. There are several factors which affect fit and comfort just in the bicycle, then several other factors in the rider. Reach and stack don't account for effective top tube length, stem length and angle, seat tube and head tube angles, saddle and handlebar height, bar type (drop, flat, trekking, etc), handlebar geometry (drop and reach of a drop bar, the degree of bend of a flat bar), fork steerer tube length and associated height of headset spacer stack, etc. By themselves, stack and reach figures are not terribly helpful in resolving OP's question of "what size to get".