I've always bought multiple BF's & let them pair off on their own..............they were in the 3-4" range.
I"ve done it with Semi's & Pakastani's.................they will pair for "life" if done this way.
If you are trying the less expensive species I'd go that route, but be prepared to have alternative homes if/when the others are bullied.
I would'nt bother trying two unless I already have one that's established & wanted to add a mate.
I'm not sure, but I don't think BF's change sex like clowns, anthias,wrasses.............so I don't think the size idea would matter or make sense. If I'm wrong then I guess the large & small(juvi) route would seem logical.
Another thing to consider is that BF species have different behavioral characteristics. Some are loners unless in breeding mode, others are always in pairs & yet another type are seen in groups. I'd research which type of behavior is expected & lean towards the grouping BF's.............they should be the easiest to pair.