I used square tubing, Mainly because I got a lot of it free, and the rest I sourced from the remnant table at the metal store. You will be really surprised to see what ends up on those tables, I have built a lot of stuff off of those tables for cheap,
as far as design, mine is a hybrid tower system. I wanted a few things, so I just sat down and drew designs till I got the general idea, then started welding.
I wanted my brew kettle up high enough to gravity feed into my fermenter
I did not want to have to handle a mash tun full of grain higher than 4 feet
my hot liquor tank was going be below the mash tun so my center of gravity was low.
Mine is a 5 gallon system, I use 8 gallon kettles and a 10 gallon igloo for the mash tun. Am devising a herms system for it right now, I honestly think I could fit in bigger kettles for a 10 gallon system but would want a wider footprint for the weight.
and that is why you should look at a lot of designs in some of the DIY threads here and go for it
My foot print is 15 1/2 by 44
biggest advantage I had while building mine was I was able to easily change details while I bought my equipment. I was not set into a situation that I had to buy for my design. I was able to save a lot of money by shopping hard for deals.