If there are no other constraints, just leave it to the fabricator to pick most economical dimensions.
If you are the fabricator, then try it several different ways and see what works out cheapest and you'll know next time.
In some cases, it is advantageous to make the shell length or the circumference match up to plate dimensions that are readily available.
In some cases, you'll want to hold the thickness small enough that it doesn't require stress relief.
Head suppliers may have various limitations as well.
Crane capacity or overhead room or rolling capacity could all enter in, in some cases.