Sorry if this has been asked before, but I would like to know if there are any rules regarding large weapons. (Keep in mind, I only have the core 5th edition rulebook).
Since I couldn't find anything, I just add the weapon's size modifier to all of its stats, including minimus strength required to wield and and to its load. Is that reasonable?
Hi,
Probably not.
A giant's great axe, or even an ogre's, should be pretty useless to a man of Size 0 and Str 5, no matter what the stats say.
That axe, which iirc has a Defense 0, probably doesn't offer the giant any extra defense.... but might well offer more Atk and Dam than the difference in size might allow.
A large net probably shouldn't start causing damage.
And so on.
If we're talking about larger weapons but within human range, I recommend leaving things as they are, at least until the combat rules are rewritten. (For the next edition at the latest, if tradition holds true. 
)
Anyway,
Ken
I think you have to be careful here just adding those numbers like you did. Let's take a Regular Greatsword vs a Greatsword that you need to be size +3 to wield.
Init Atk Dfn Dam Str Load
Regular +2 +5 +2 +9 +1 2
Size +3 +5 +8 +5/2 +12 +4 5
If we assume that each creature has the required strength to wield the weapon and their score in 2hd weapons is the same. I am also assuming Dex and Quik are the same, for the sake of demonstration.
Initiative - probably no worries here since the weapon is larger/longer---they should attack first.
Atk - It is a bigger weapon, so maybe no worries.
Dfn - The weapon is larger, but it is covering a larger being (since you need to be +3 to wield it) I would leave it at +2.
Dam - I am increasing the damage it does, since the weapon is bigger, which makes sence.
Str and Load - Both these are fine
Now, given these numbers, the Size +3 weapon will:
Attack earlier. (+3 to init)
Hit more often. (+3 attack)
Be hit the same. (+2 Dfn)
Do more damage. (+3 attack and +3 damage and +3 str (or more))
So that's +9 more damage, which will seriously sway the battle.
Now this can be compensated for by lowering the weapon skill, Qui and Dex of the mob, which in turn will:
Attack at the same time or later.
Hit at the same rate or less.
Be hit more often.
Still do more damage.
Thus, you need to tweek all these numbers so the fight comes out fairly. So, just as long as the mob wielding the weapon gets the added damage from having a high strength, things should be ok. Remember, these rules were written for fast and easy combat: not to have all situations fleshed out. It would be great for an official supplement to come out with revamped combat rules, but until that happens, I think you need to stick with what we have so you don't unbalance the fights.
If you're going to change the weapon's attack and defence values, you should probably change them an equal amount - after all two giant fighting each other shouldn't hit more often than 2 dwarves... (at least not as a result of the weapons).
However, adding to the damage stat seems fair, as larger beings have larger wound ranges.
I'd probably just add size to the damage, and half size (rounding down) to initiative (initative makes up for the reduced quickness larger beings get).