Don't fee ashamed mate we all have to row with the oars we were given and a small pond is better than none at all. Look at all the loons we have here with growing on tanks of 1000 gallons or less.
Looking at the measurements you gave you pond should me close to 1000 gallons or 4,5 tonnes. Yes it is a bit small but with a sensible stocking level and regular water changes you should be ok.
Don't go beating yourself up over having a liner pond. I have one and am proud to say so. I would be happy to give you a bit of advice on building a pond as you describe. Here in Holland this is a very common method of pond building.
First off some comments on the materials. Many people here use plywood as a retainer walls for their liner ponds. To be sure not and old plywood will do. Commonly we use shuttering board. This is plywood with a black/brown oiled outer layer that is held together with waterproof glue. It is very resistant to rotting and will last much longer than anything else you might try made of wood A second alternative that I used were foam sandwich panels that are used for making cool cells (cuttlebrook made their tosai house out of them) and with a bit of angle iron in the corner then hold quite well.
Before you run out and buy either of these look around and see if you can get some 2nd hand. I have bought both shuttering ply AND sandwich panels 2nd had quite cheaply off the Internet. The sandwich panels I got for free and the 25mm 18-ply shutter-ply were a tenner apiece for ply that was 5'x11'! With ply this size you could go a bit deeper as well.
Realize that when building with this method the wall serves more to keep the edges of the pond from collapsing into the hole rather than retaining the water and shape of the pond. it is extremely important that the back filling of the outside edges between the yard and the wood be done carefully to avoid the walls of the pond being pushed out of shape by the weight of the water of conversely the walls of the pond being forced inwards by the pressure of the back fill.
When making a pond with plywood walls need to be supported with posts ant every seam between two panels and preferably with several in between. It is inadvisable to use treated wood for this purpose as it tend to rot anyway in a very short time. Spend the extra cash and buy tropical hardwood posts that will last as least if not longer than the plywood.
With this kind of pond you don't need a solid bottom. Mine is simply sand with the bottom drain buried in the sand. You my pour a floor if you wish but I see little point What you do need is to make sure the bottom is covered in CLEAN sand and nothing with rocks and pebbles in it.
My one comment is about having the pond raised above the level of the yard 2'. If you have a wood walled pond then as I said before the back fill is what is holding the pond walls in shape. The more you raise the pond above the surface of the yard, the greater the pond will distort and need extra re-enforcing. This is also a weak spot that may fail in the future of wood starts to go soft from moisture and age. I would sat restrict the height to a foot at the most. and I would put an extra layer on the outside along the top to increase the strength.
As far as the liner goes, fold the edge behind in the corners and glue the seam with some of the special mastic they make for epdm rubber. Should this leak then all that happens is you end up with a bubble of water behind the fold but no leak.
[img]http
://www.dutchkoi.com/picture-files/deep23.jpg[/img]
A pond from sandwich panels.
[img]http
://www.koi.excalibur-nw.com/images/sandwichpond.jpg[/img]
I placed mine horizontally instead of vertically but it is a sort of hybrid as well since I later extended the bottom downwards using shutter board as seen in this picture.
Top (green/black) is sandwich panel
Bottom (grey/black) is sandwich panel
[img]http
://www.dutchkoi.com/picture-files/deep12.jpg[/img]
Frame for a small shutter board pond
[img]http
://www.koi.excalibur-nw.com/images/betonplexpond1.jpg[/img]
Same pond with ply in place and a portion of the insulation
[img]http
://www.koi.excalibur-nw.com/images/betonplexpond2d.jpg[/img]
More height above the yard means more support
[img]http
://www.koi.excalibur-nw.com/images/asobepond.jpg[/img]
B.Scott