Hey Ando, welcome to CB!
And from the sound of it, you're already at the point of being ready to roll and (hopefully) without any teething problems of a new build. Something always seems to crop up of course, even with ground-ups, but hey, we'll hope for perfection.
Shoot us some progress pics when you can sometime.
Regarding tire sizes, there are tons of discussions here, so when you get some time you might want to cruise through the backlog to see if you glean any new info, but I'll give you the basics here.
Wheel size and offset/backspacing is critical, especially for the larger sizes. In that vein then, usually a 31x10.50x15 is typically considered the largest you'd want to mess with on a stock Early Bronco. There are taller sizes, but not many wider that will fit.
There are modern metric alternatives to the stated "flotation" size above, such as 235/75 15 or even something similar in a 16" wheel size as well. Look for equivalents to the range of 28" to 31" diameter, and roughly 8" to 10" wide or so.
Narrower wheels are best for ease of fitting, but you can go up to 8" wide with a 10.50 wide tire, with the right offset.
I typically recommend a 6" or 7" wide wheel, with most sizes however. Even up to 10.50's, a 7" wide wheel will be more likely to tuck in under the rear fender. And that's the key. The rear is the tighter fit over a tire than the front is on these trucks. You might rub the front radius arms with the larger sizes when turning sharply, but that's not hurting anything other than your peace of mind when you hear that rumbling from up front and think something on your new rig is falling apart!
That's about all I've got. The backspacing and offset numbers are what will keep the outer edge of the wheel tucked in far enough to clear the rear sheet metal.
I'm sure others will chime in as well, but in the meantime check out some of the other threads that I'm sure include some images of member's Broncos that you can use to decide what you like.
Good luck. And have fun!