A magus wants to cast a spell with target room, affecting the room he is in.
What range is this? And why isn't it range personal?
Presmably nothing changes if it's a device that casts the spell, unless perhaps if the device is the room?
Range Personal would allow the room to affect itself, such as an enchantment on the room allowing the walls to harden (in case of attack). I would use range Touch if the person/device will be in the room when casting the spell.
Chris
No, if you'll read the description of "Personal" on page 111, it specifically states that it cannot affect a target greater than Individual. It doesn't really make sense to me either, but them's the rules... 
Stupid rules*! Marcus of Criamon has learned the Spell: "Eyes from the Wizard Torn" (see MoH). It's a range Personal spell. Of course he can't cast it on himself, because he is Giant-blooded (size+2 > Individual)!
*in this case
beautiful!
And thanks for the replies. Serf's parma, shold have mentioned I guess
The real reason is because, as the Personnal description states, "The spell only affects the casting magus or things he is wearing or carrying. The Target is thus never larger than Individual" (ArM5, p.111). A Range: Personal spell cannot affect something else than the caster, that's the reason why. Not because it can't affect something bigger than Individual.
We could deduce from this rule that the target could be bigger than Individual, if the "caster" is a magic item the size of a room, for exemple.
True. There's also the issue of what happens in cases where multiple individuals are considered to the be the Caster such as Wizard's Communion, or Ceremony...
In wizard's communion, there is ONE caster who actually cast the spell.
Ceremony is not hermetic so no problem.
Yet if you look at the rules for target sizes you'll see that intellego spells are not constrained by the individual size of their targets only the category. (is "Eyes from the Wizard Torn" an intellego spell)
He's still Target Individual, and the spell is target Part, which is higher level than Individual, but smaller, because it's part of an Individual, specifically Marcus.
To go back to the original question, the restriction on Personal Range is supposed to stop you affecting a Room at that range.
Maybe a little unclear.
Hmm. Muto corpus. I guess it is affected by target sizes.
Of course what David says certainly applies. Target part, from the box on page 115, can affect items up to the same size as target individual. For corpus the base individual is size +1, so unless Marcus's eyes are the size of a horse he'll be fine with the spell.
(ergo the "stupid rules" comment is in this particular case unwarranted.)
I apologize for being overly harsh.
I still find it interesting though that giant-blooded magi are limited when it comes to Personal spells (which are immensely useful - not just because the level is lower but because they can be cast free of penetration, so one's parma is no problem)
They aren't. The Target is still Individual, just with a size adjustment.
A big individual is still an Individual.
You are right that a vanilla Personal Range, Individual Target Corpus spell will not be able to affect a Giant Blooded character. But, a Personal Range, Individual Target Corpus spell with a +1 size modifier will be able to affect a Giant Blooded character.
The second spell is still a Personal Range spell. It is still a spell with an Individual Target. It just has a size modifier to allow it to affect big individuals.
And that size modifier means the spell needs 5 more in casting total to work. Ie. a limiter.
(not saying its wrong, just noting that it IS a limitation)
Yes, that's true. Adding a size modifier increases the magnitude of the spell.
If it is a bother, you could work into your saga the nearly forgotton research of the tenth century magus Fezzik of Flambeau who integrated the target of "Self" into Hermetic magic (same level as individual it can only target the caster, but can effect targets of up to size +4). Fezzik was a school of Ramius caster who routinely increased his size to face supernatural opponents.
the character reads a book and suddenly there is no longer an issue.
And yet, it is not big a limit, since you design your spells for you. Compared to the advantage of having some of your opponent's spells fail against you without him being able to do anything about it, + the added fatigue levels for spontaneous magic, this is absolutely nothing IMO
There are no added fatigue levels in fifth edition.
Oups! 
Senescence striking back again 
I guess that made the virtue too powerfull.