they grow surprisingly slowly IME. the zigzag eel i've had for awhile doubled in size the first year i had him...from a tiny guy of about 2" to a little over 4". in the next 1.5 years or so, he's only grown another 2". so right now he's about 6-7". i do believe this eel might be of a species that stays generally smaller...so he might not be a good barometer for you. my tiretrack was purchased around 4", and he has doubled that in a year.
identifying spiny eels at the species and sub-species levels can be tricky at best and impossible at worst...and only the fire eel and peacock eel seem to have been completely pinned down and agreed upon in books and throughout the web. in fact, even the peacocks have substantial variation, and there may be subspecies. the zigzag and tiretrack eels appear to be a mixed bag of closely-related species with similar needs, but it isn't always easy to figure out the eventual size. you can estimate, but that's about it! at this point, i'm figuring the sky's the limit on my tiretrack...especially after doubling from 4 to 8". i expect him to get HUGE. my zigzag, on the other hand...i wouldn't be surprised if he topped out at 1'. we'll see. you can expect the fire eel to grow along the lines of the tiretrack, but top out at an even longer length.
as for tanks, the 90g should be fine for both i would think, though a 125g is a glorious tank and they'd love it if you need an excuse! yes, these "eels" get huge...but they don't run into the logistical problems of more typical fish. meaning: a 20" gar can't turn around easily in a 18" wide tank...a 20" spiny eel can. i wouldn't keep them in anything less than a 55g even when small, but any 18" wide 75g+ i would think could work.