I have to agree fully with Carl, I myself will take on just about ANY project, I have built cars from nothing, bought and built my own AG pool, Built many decks, fenced many yards, shingled many roofs and added onto many houses without worry.
BUT, when it comes to this AG pool, having only a wall does not make for a good start, your associated costs for an AG pool are not in the wall, rather in the equipment required to run it, the wall is worth about 1/10th of the whole pool. By the sounds of it you will save another 1/10th or a little more by not putting in the lower track, making your own uprights and top rails, but will have cost incurred for the lumber needed ofsetting those savings.
Burrying an AG pool will rot it prematurely - if it is a 30 yr wall you will get 10 out of it - soil is acidic and will eat the wall quickly.
If you would like to have a guestimate of the pool size without opening it, just take a string and follow the rolled up metal with it and measure the string needed to go from end to end of the metal - that will give you your perimeter, then divide by 3.14 to get the diameter.
If you're only going to put top rails on it and no uprights, one person pushing on the top rail to get out will give you a tsunami across the backyard.
Once you cut your sem cleanly, you will have to crimp it for strength otherwise it will rip right off the bolts - also rustproof your cut end(s) - plain old primer won't do much to protect it, electroplating may be a better idea.
As for round pools being rolled in a circle and oval not - how else are they to roll up a wall - square??? (sorry, but I had to laugh at that one). I've seen oval pools pre assembly, the wall is still rolled up round....it's the bottom track and top rail that then pull the wall semi straight again.
It's a big project to undertake building an AG pool (well, under 20 feet isn't bad if you have all the parts, but bigger pools require a LOT of hard labor), even bigger if you're also manyfacturing your own support structure, if you decide to build this thing, I, like Carl will sign off here and wish you luck!
Peseverence and hard work may get the job done, how long it will stand uprightis anyones guess....
As for liner sizes, if you're cutting off a few inches, the same size liner will work, however, if it's a few feet then you're better off taking it down to the next smaller size available by cutting it to the proper perimeter for that smaller size.
Good luck!