Glock size

Yes, it's noticeable.

But what do you want they gun to do for you?

Depending on the task, one is better suited than the other.

I had a 26 and sold it to get a 30sf. The 30sf is very bulky, heavier and bigger. I sold it 2 month ago and bought a 27.

The G30 is more akin to the size of a G19, only thicker.

The G26 is a bit smaller:

Grip size between the G26/7 and G21SF is just a hair difference. I dunno if the 30SF is somehow different from the 21SF, though.

IMHO, unless you feel like you are really compromising your grip with the G26 to where you can barely handle the gun, you should be comfortable with the G21SF.

According to my calipers, the reach of the 21SF from backstrap to the trigger is only 4 hundredths of an inch longer, and the grip is only 1 tenth of an inch thicker. And the hump is much smaller, too.

G27
grip
http://i688.photobucket.com/albums/vv241/gloob27x/GLOCKSIZE001.jpg
trigger reach
http://i688.photobucket.com/albums/vv241/gloob27x/GLOCKSIZE002.jpg
overlap
http://i688.photobucket.com/albums/vv241/gloob27x/GLOCKSIZE003.jpg

G21SF
grip
http://i688.photobucket.com/albums/vv241/gloob27x/GLOCKSIZE004.jpg
trigger reach
http://i688.photobucket.com/albums/vv241/gloob27x/GLOCKSIZE005.jpg
overlap
http://i688.photobucket.com/albums/vv241/gloob27x/GLOCKSIZE006.jpg

The difference between the SF and non-SF is just another hair or 2. It's really an individual thing at that point. You can save some money on a used non-SF if the glove fits.

I have a G26, it fits me well, and I shoot it well.
I've held both a G21SF and a G30SF, both had a trigger reach that is just too long.
So the difference might not be much, but it is there.
If your hands are bigger than mine, you probably won't notice.

Well, with grip size, you have to measure circumference rather than length or width alone.

My G21 SF is a little bigger than the G27--noticeable but not hideous like the old G21 was. It is remarkably heavier, and that's why I don't carry it (I carry a G22 or G23 these days)

I have owned most of the glock models and currently own and carry depending on whim a 21SF or 23 of the baby glocks (sub compacts) I like the G27 the best...but to shoot it well I need a grip extention on the mag...which pretty well makes it a short barreled 23 in size....so I just carry a 23....the 30 feels great to me in hand...but like the 21SF it's a wide and bulky gun...not ideal for IWB though I know several that do just that with it...it's a solid gun in a great caliber....but I think the 19/23 is really the ideal carry size for a serious CCW...if you want to make a concession to size/carry the 26/27 are nice to tote...if you have medium average hands you might love it...if you have big mits the 23 might feel plenty small! It's all what fits you and how you want to carry it....by and large a 19/23 in a strongside holster of quallity is hard to beat. the .45's are wide and beefy but I LOVE my 21 SF...shoots like a house on fire!