Seemingly complex answer, but once you figure it out it's pretty easy (like a lot of RM).
Your arm length is roughly half your body height... minus the size of your chest, minus depth, divided by two.
So, just to make it easy (I'm going to use rough/average numbers), let's say a 6' man has a 72" span. Divide by 2 and you have 36"
He has a chest of 46" around. Let's take 12" off for depth (34) and divide that by 2 (18) has a 18" chest span divided by 2 = 9".
72/2=36" - 18/2=9" all totals 27" reach.
Take a weapon, and just for ease, say you use 6" of the weapon for grip.
So a 36" sword, -6" for grip (I'm not going to worry about 1h vs 2h difference), +27" each gives you a
max range of 57" or 4.75 feet to just barely touch an object (if a character is trying to poke an inanimate object that might come up). Let's take another 3"-9' off just to say you can effectively land a hit that matters on a foe.
So we'll just call it 4 feet in this example.
Basically, if you're using 5" hexes/squares, you can reach the ones next to you because I'm going measure from the center of your hex to the center of theirs. From there a weapon that is 5 feet longer (9+ feet or more) will give you another hex/square. (I use 5 feet hexes in close combat).
In the end, you figure the well built 6' character has a reach of
15" + weapon length and I believe most RM materials have weapon lengths listed.
If you play around with numbers you'll probably come to the same conclusion a lot of designers do: A character will need to be significantly differently built to have a big enough impact on reach to take it into account in most situations. In the end I think you can just apply common sense. 1H weapons (and even most 2H) are a 1 hex reach and things like Halbards, Spears, etc, you get one more hex.
But... some day that situation is going to come up where your PC wants to push a button, flip a lever, pop a balloon, etc.