Printing: custom size, full bleed spread Epson Mac settings?

[NickBurchell]

I'm on a tight deadline and hope someone can help.
Photoshop CS3
Mac 10.5.8
Epson 3800
Moab Entrada paper, custom cut by Lost Luggage for screw post portfolio's. Paper is 11" x 11.75" - image area 11"x11", with 0.75" for the screw post.
I'm printing spreads with bleeds - images that go across both pages and bleed off top, bottom and side, also extending slightly into gutter.

My problem is that I can't seem to get the custom page size to bleed accurately. The image page (photoshop file) is sized 11x11, but is printing 10 6/8ths x 10 6/8ths. I'm not sure if it's Page Setup, Photoshop Print Preview or the Epson Printer driver, but something seems to be resizing the file image to print smaller than required.

I'm wondering if this has something to do with Page Setup (which is part of the Mac OS I believe?), which has always confused me with the multiple print size settings - ie for 11x14 bleed printing (borderless) there are several options, two of which are:
Borderless, Auto Expand, 11x13 or
Borderless, Retain Size, 11.39x14.39

I should also say that image resolution is 360 DPI, printer native resolution, which I'd like to keep printing at. If I change the scale size in Print Preview, Photoshop adjusts the DPI accordingly.

To make this more difficult, I'm running low on ink for testing (even rush ordering is not going to get any to me in time) and the custom cut paper is very expensive to be experimenting on, so I'm trying to figure this out with a few test prints as possible!

Thanks for any help!