With the rise of position less basketball it doesn't really matter. I have personally switched to looking at players (particularly draft prospects) as Guards, wings, big wings and big. Guards being players who are pretty clearly primary handling point guards (Irving,Rozier,Larkin), wings being every player who isn't a primary ball handler but who can be exposed in the paint on defense(Brown,Tatum right now, Hayward,Nader, Bird), Big wings being wings with the size/strength to defend traditional bigs in the paint (Morris), and Bigs (Horford,Monroe,Baynes,Theis). Like any classification this isn't perfect. Smart is really a hybrid wing/guard.
As for Tatum if the Cs are going to keep a similar roster construction next season then they will likely start Irving, Brown, Tatum, Hayward, and Horford. This will give them possibly the best shooting starting 5 in the NBA. It will however require one of the wings to play the big wing role. All 3 wings have the potential to fill this need.
Hayward is 6'8 and probably at least 230 lbs but with only a 6'8 wingspan
Brown claims to have grown to 6'8 237lbs and has a 7' wingspan
Tatum is a a bit on a mystery in 2016 he was measured to be 6'8.25 204lbs with a 6'11 wingspan. On the court he seems taller and especially longer then this and the rumor has always been that he is still growing. For arguments sake lets say he is 6'9 with 7' wingspan. I suspect Tatum will have a productive off season in the weight room and will begin to fill out his frame.
To answer the original question with weight gain Tatum will be best suited playing the big wing role in the Harrison Barnes GS mold. His inferiority to Brown on the perimeter and height and length advantage over Hayward make him the prime candidate. If he isn't strong enough next season we may see Hayward still take on the role short term. With so few teams looking to post up the Cs will have the benefit of having three very switchable wings who can all abuse slower matchups on offense.