It partially depends on the variety. Most of the fancies don't get much larger than 10 inches, even in ponds, but commons, comets and shubunkins can get much larger and will outgrow anything smaller than a 55 pretty quickly.

If you plan to keep your Goldies as aquarium pets, with 8 of them already at 4 inches, you will need at least a 29 or better, a long 30 for them immediately, so unless you have plans for a second tank, you might as well get as large a one as your space and wallet can support, but nothing less than a 55, with double the filtration ordinarially used for that size.

Because they are large, bulky, cold water fish, Goldies need far more tank size, oxygen and filtration to be kept healthy than do any tropicals.

Goldies are beautiful fish, but unfortunately, because they are so inexpensive, don't usually receive the respect they deserve, but if you keep them as they really should be and can give them a 125, they will be able to grow to their true size and you could have a gorgeous set up, with fish that will become so tame, they will allow you to literally pick them up to cuddle.

Then, as a bonus, they will live for many long years. Given the right conditions, carp are quite long lived.