It means you have 10ft reach, and can benefit from large sized weapons, though you can stil use ordinary medium size weapons if you have to.
Also makes it a bit harder for you to be grappled or tripped, makes it a bit easier for you to grapple or trip others---there are size limits.
There's also a bonus to Intimidate for being larger than your opponent, for what it matters.
It does not give you reach. While reach if often related to size, it's not inherent in size changes.
It also applies for combat maneuver checks, intimidate bonuses, being blown away by strong winds, special attacks like snatch and swallow whole, etc.
Being big is a really powerful boon. You gain bonuses to combat maneuvers, gain reach, can use manauevers on larger creatures, can take the Awesome Blow feat, can wield larger weapons that deal more damage, etc. The only downsides is that you have slightly less AC and it's harder to maneuver.
It does not give you reach. While reach if often related to size, it's not inherent in size changes.
It also applies for combat maneuver checks, intimidate bonuses, being blown away by strong winds, special attacks like snatch and swallow whole, etc.
What do you mean? Large creatures have a natural reach of 10 ft.
Plus I guess this is a homebrew campaign, so classic conventions don't hold and if the DM says so, he has whatever reach the DM says.
Assuming 10' reach (on which I agree with the interpretation of others here), Cleave and Great Cleave become more useful to you.
What do you mean? Large creatures have a natural reach of 10 ft.
Not necessarily. Depends on if they qualify as Large (Tall) or Large (Long). Large (tall) creatures, like an ogre, have 10' reach. Large (long) creatures, like a horse, only have 5' reach.
Being large doesn't automatically give you 10' reach. It depends on what form your body is.
That being said, the standard player races would qualify as Large (tall) once enlarged, and would have 10' reach.
The half-giant race basically has this trait, called powerful build.
In that case it's pretty specific what it does and doesn't do, and is probably a very reasonable guideline.
Powerful Build: The physical stature of half-giants lets them function in many ways as if they were one size category larger.
Whenever a half-giant is subject to a size modifier or special size modifier for a Combat Maneuver Bonus or Combat Maneuver Defense (such as during grapple checks, bull rush attempts, and trip attempts), the halfgiant is treated as one size larger if doing so is advantageous
to him.
A half-giant is also considered to be one size larger when determining whether a creature’s special attacks based on size (such as grab or swallow whole) can affect him.
A half-giant can use weapons designed for a creature one size larger without penalty. However, his space and reach remain those of a creature of his actual size. The benefts of this racial trait stack with the effects of powers, abilities, and spells that change the subject’s size category.