Model size does not necessarily translate into more reach in this game. For example the Leviathan is waaaaay bigger & longer then the Calibur, but the reach is exactly the same (personally tested).

In this case I am not sure, but later tonight I would be happy to drop by the ATH & test Twisted Snarble Barb 3* vs BTB 5* and let you know if there is any reach difference (not exactly what you asked, but if the reach is the same then DTB 4* being the same is a safe bet).

Do you have sealed sword to test reach of DTB vs Sealed (any of them) with? I remember that the Sealed Sword, DA, Faust and Troika all have the same 1st swing reach (personal tests). To test, equip both weapons and find a regenerating destructible object (blast cube or whatever). Pick one weapon do first swings at the object until you find the maximum distance you can stand from the object and still break it with only the first swing (no step). Then switch to the other weapon with out moving and see if it will break the object too. If so see if you can get further away with the other sword and switch back and forth until you are figure out if there is a reach difference or not.

I don't remember for sure on this one but I want to say that the BTB has the same first swing reach as the Troika & Sealed sword. I would not swear to this though, so I would have to test it to be sure.

As far as what to upgrade first?.. If we are just talking about LD as a striker (unless you are highly skilled), go ahead and prepare to have your rear end handed to you until you have a fully heated: Skolver set, BTB/FF, and 2x 5* Heart trinkets. I know somebody will come along a flame me saying something about skill, Recon, bombing, gunning, or something that is why I said "unless you are highly skilled" (said it twice & bet the flames still come). Honestly my LD recommendation would be to make yourself a competitive 3* T2 LD loadout & use that to earn coins until you can get a good 5* T3 LD loadout (including 2x 5* heart trinkets) together. This worked great for me.