gonna go into DK for a second here:
VS
http://cghub.com/files/Image/003001-004000/3851/203_realsize.jpg
(page is stretched enough, click the link to see it)
if you compare the two, DK seems to sport the musculature of both humans and Gorrilas.
judging by the way his shoulders and pecs are shaped more like a human's rather than a gorilla's, Id say it could be safe to say that this ape can swing a punch much lieka human could, and with those pecs, triceps and back mucles, being hit is something everyone should fear from him.
Combine the fact that according to almost all media he is over 6feet:
(cartoony, yes, but pauline = peach essentially, and peach is tallish, a good foot or so above mario who is 5', with DK being taller still...)
http://www.youtube.com/watch?v=gRO1re9jJCE
this vid (while shakey) shows a more recent size comparison, which shows DK is about 6ft even when not standing up, and just on all 4's. His arms alone take up about 5/6ths of his height, so his reach must be in the 5' area...
http://www.youtube.com/watch?v=UVWnwPml7aY
if you want another (slightly less canonical) appearance to guess height: Lil mac is said to be 5'7" in the Wii Punch-Out!
anyways, what we got here is a character with a 5ft + reach, who WILL overpower anyone essentially if he gets his hands on em, and will likley tear them to shreds with his ape-like musculature.
Now as for Roy, I am not doubting his skill with the sword, but lets look at his features for a second:
Roy is a 15 year old kid, who likley isnt fully developed musculature wise, and is depicted as
not being exceptionally tall or muscular for his age. He is probably around 5'5" and 130 lbs.
As for his sword, combined with his arm length he is gonna have a reach comparable to that of DK's own, allthough his has a disjointed Hitbox.
http://fireemblem.wikia.com/wiki/Roy
looking at his history, Roy is said to have been off "studying" when he got called to battle, and is very clever and cunning. That being said, does he actually posses proper training with the sword like Marth and Ike do? Sure he gets the sword of seals and stuff, but he also has party members and matures into a lord very late compared to the other protagonists in the FE series... Evidence that he may not be up to par Swordsmanship-wise?
anywho, thats rather controversial and bringing up backstory a bit too much. But theres no doubt he is smarter than DK (who isnt *that* dumb, I mean look at the DKC game where he has to deal with all kinds of stuff that solidifies he would have some smarts other than how to find bananas).
But on the other hand, other things show that DK vastly outperforms Roy in all other aspects besides brainpower and having a weapon on hand (unless there is a convienent barrel..).
Having just about the same reach, range isnt exactly the main issue, as both are deadly at the same range just about. So now lets looka t what each can do to each other:
-Roy can slice at DK and inflic some nice owies on him if he "spaces" right, but DK is bigger and faster due to being a phenominal physical specimen, even if he is bulky.
-If DK gets a hold of Roy, the match is essentially over as Dk can essentially toy with him all he wants with his phenominal strength. For example, grabbing his non-sword arm and tossing Roy aside could very well dislocate Roy's shoulder. Better yet, grabbing him and slamming him to the ground could not only do some damage but disorient Roy, leading to more blows and eventually death.
-Roy on the other hand can only hit the head or chest of DK to land a 1HKO, where anywhere else will either be just an injury, or honsetly shurgged off as DK's thick hair/fur, muscle mass and fat (mammal) will probably help with softening a blow to the point where it isnt "loses a whole arm" type of deal.
-if DK ever lands a hit on the much smaller Roy, or even worse, gets a hold of him, ROy is essentially screwed as DK can break his bones, bite him, or just slam him around too fast for Roy to get a good, accurate swing in.
-Having similar range, but being out manuevered by DK leaves little room for mistake for Roy, who has no history of extensive sword training other than his experience of being thrust into military service. 1 mess up from Roy means DK kills him.
-Roy could probably trick DK into getting a hit, but DK's agility and long reach could turn the tables if he say, grabs Roy's leg and trips him as he tries to attack.
TL;DR:
DK is a big ******* with comparable range to Roy. Roy's only advantages over DK are his weapon and his Cunning. DK outclasses Roy in every other aspect, and can take some hits from Roy whereas 1 hit from DK could very well mean Death to Roy, as he is Knocked over or worse: Grabbed by a giant gorrila who wants to, and probably could, tear him in half.
Thats not to say it would be tricky for DK to grab Roy past a sword being swung, even if he isnt an *expert* like Marth or ike