A pond is a personal choice, from either space or funds!

My advice is simple, make it as big and as deep as you can afford in cost and space, because the bug WILL get you sooner or later and a small pond will haunt you till you make it bigger!!!

as for a the sides, straight sides are good, as long as the bottom slopes to a sump so the waste settles in one area, easier to remove than chasing it all over the bottom of the pond.

Let me say from the outset shelves provide easy access to your friendly heron and maybe even assist mink if you are cursed by their presence. I have had a heron stood on the plants/plant-pots on my 2ft deep shelf and at 70cm deep your pond will, most likely, not be heron proof. My fish pond has a 4ft deep section that, I think, saved my fish form the heron. That said I wouldnt be with out shelves, you pays your money and takes your choice.
Assuming by straight Alan means vertical I would ammend his last paragraph to read " .....straight sides are good, IF THE SOIL WILL TOLERATE THEM, as long as......".
With regard to shelves, what sort of plants are you thinking of and is the pond to be wildlife friendly, if the latter then I would suggest additional shallower shelves.
Shallow shelves especially with a low surround help any animals that fall into the pond escape and very shallow shelves are baby amphibian friendly. My shallowest shelves, in ponds 2 and 3, a wildlife pond and a plant pond, are between 1" and 3" to the liner. In the wildlife pond the plants are planted pot free in garden soil. Some of this soil broke the surface but last autumn I did a bit of tinkering which deepened the pond by an inch so I may have to add some gravel to get back the through-the-surface shelves. Garden soil may have been a mistake as it adds nutrients to the water and feeds blanket weed.
Last autumn in an attempt to reduce blanket weed in my fish pond I reworked the 1ft shelf so that the plants are pot free in gravel beds. The idea is that the soilless planting will cause the plants to draw nutrients from the water, thereby combating blanket weed. I think Sue, Scotty, does this.
I am going potless because I hate ripping roots off plants in order to repot them. Going potless I hope not to have to lift them at all. The MAJOR drawback with this is that over time I expect everything to grow together and my gravel beds to become 'one'. If this doesnt suit dont consider potless planting.
If you do consider potless planting to be a possiblity it would be an idea to 'lip' your shelves with raised edges as these will help retain the 'media' on the shelves. However be aware of plants with sharp roots or that spread by runners. I have yellow flag iris contained in a soild wall pot in the fish pond and will be lifting some of the reeds and rushes in the wildlfie pond and putting them in solid walled pots too. Soild walled pots = cut down washing up basins etc.

With regard to the depth I would try to go deeper than 70cm if you can, heron protection. A shelf or area at 60cm is not a bad idea for lilies, water hawthorn and maybe even golden club. The more vigourous lilies can grow in water deeper than 2ft but hawthorn and the like would probably struggle.
Hornwort will grow in any deep depth as will most of the planted oxygenators

mine is 3ft deep in the most part,,and i get the bloody heron every day just now,,but never even attempted to get in there,i think it must be too deep for it,as its emptied the pond in the street twice now!
i have straight sides too,and a sore back ;-) i got the bug from a small "inherited" hole,,and yes,,the bigger the better,mine is about 3metres from the patio and i love it...

As i said in a previuos post i am going to make my pond larger and deeper in the deepest part. I am pleased to hear that some of you are planting direct into the soil and not into pots, i intend to make a 6ins planting ledge on three sides of the pond, the intention is to build up a small hump on the 3 sides before laying the liner and that will give me the planting shelf that i require, my soil is heavy clay so should be ok, any sujestions would be welcome.