When starting work on a large unit cell it is wise to start with a DZ basis. With such a basis, any of the above suggestions can be tested. However, for most properties, the DZ basis is probably not very accurate. You can next go for the TZ2P basis set, but that may be a bit of overkill. One reasonable trick is to remove f polarization functions (if present) from this basis set. So for copper you can comment out the 4f STO
AtomType Cu
DIRAC Cu
7 5
VALENCE
1S
2S
2P
3S
3P
4S 1
3D
SubEnd
BasisFunctions
3D 1.280000
3D 6.900000
4S 0.850000
4S 2.450000
4P 1.370000
! 4F 2.200000
SubEnd
FitFunctions
1S 48.900000
...
Note that this will save you 7 functions per atom. If this is still unwieldy, you can also decide to start from the DZ basis and add a polarization function by hand.
