Tiny ARM Development Board?
After measuring a Raspberry Pi and looking at claimed dimensions I conclude that your 25mm measurement refers to the "height" or thickness. 

A Raspberry Pi model B is slightly less than 25mm thick between extreme projections.
 (4.5mm SD card socket + 1.6mm PCB + 15.6mm dual USB connector = 21.7mm)

The model A may be slightly less, and you can easily reduce Model B height by 2.3mm by fitting a single USB socket or hard wiring to an external socket.
 You can reduce height further by removing the "ethernet" socket and further again by removing the RCA socket. BUT you said &lt;= 25mm so the Raspberry Pi model B should work as is. 

Raspberry Pi Model B is available On ebay now for from $105 buy now.
 Or I'll send you one of mine tomorrow if your offer is good enough :-). These are scheduled to drop to about $US35 in the next few months.

The Olunuxino ARM board is close to Raspberry Pi and can be had on ebay for $63 buy now



Raspberry Pi dimensions:

From here: 



Photo below from here



You don't state the other dimensions which are critical.
 As you only mention width, and say that RPA (Raspberry Pi model A) mis perfect for your requirement then RPB is also perfect.

Your 25mm breadth (= width) does not make sense as a RP is    

~ 85 mm x 56 mm (PCB) or
 ~ 95 mm x 65 mm  (outre dimensions, model B)

Model B is just under 25mm thick or tall from SD socket on one side to top of dual USB connector on the other.