I am wondering if you consider them close enough in size as to be similar, or do you feel the G30/36 is more akin to the G23 sized pistol?

I haven't been able to put them size by size in a store or gun show to get a good feel for the difference.

Here's the specs on the 3

GLOCK 27 Caliber.40 Length (slide)        6.29 in.    160 mm Height                4.17 in.    106 mm Width                 1.18 in.    30 mm Length between sights 5.67 in.    144 mm Barrel length         3.46 in.    88 mm GLOCK 30 Caliber .45 Auto Length (slide)        6.77 in.    172 mm Height                4.76 in.    121 mm Width                 1.27 in     32.5 mm. Length between sights 5.95 in.    151 mm Barrel length         3.78 in.    96 mm GLOCK 36 Caliber .45 Auto Length (slide)        6.77 in.    172 mm Height                4.76 in.    121 mm Width                 1.13 in     28.5 mm. Length between sights 6.18 in.    157 mm Barrel length         3.78 in.    96 mm

Looks like if you "average" everything out, the 27 is slightly smaller than the other 2.  Now, the size difference is quite negligable, and they all are "basically" the same size.  The 30/36 is in the same class as the 27, and not the 23, but the 23 is no more than a 1/4-2/5 of an inch bigger.  

I'm a big fan of them all.  you do get a few more rounds w/ the 27 though.  As long as you're not set on a certain caliber, I'd say go w/ the 23 or 27.  27 will be slightly easier to conceal than the 23, but not a whole lot.

I got a chance to put a G36 next to a 27 and a 23. The 36 and 23 barrel length looks almost the same, but the height of the gun is shorter.

The 30 with the 9 not 10 mag would make it much shorter, but still overall the 27 is a more compact gun.

Get both...
M4-AK

I have owned all 3. I still have the 36 and the 27. The 27 was my first CC auto. The 30 is one chunky handgun. Too fat in the grip and too wide overrall.

The 36 is great, but I can forsee situations where 7 rounds before a reload is cutting it a little close.

The 27 goes with me when I can't carry my 23. I'll probably trade the 36 before long.

I sold my 26 when the 36 came out. Because I thought it was going to be my carry gun. Well after carring it for 2 months I found mysaelf missing the babies. So I bought 27, still have my 36 It is the most accurate of all of my 7 Glocks (unusally tight for a Glock) including my 21. I think the 36 carries more like the 19-23.