Whats the longest legal length in CA? What if it is registered and tagged in Arizona and I am driving in California, would it still be a no no if its over?
The answer to your question is - "It depends".
Longest single vehicle length RV you can drive with a standard Class C (C = Car) license here in CA is 40 ft.
If you live here, and want to drive a 45-footer, you've got to get a non-commercial Class B (B = Bus). Or a Class B CDL if you'll be driving commercially. Be aware that with a 45-footer, CalTrans has restrictions on certain roads for these - including the incredibly scenic Hiway 1 from basically San Simeon just north of San Luis Obispo up the coast to Monterey. Matter of fact, it seems that the majority of the most spectacular scenic roads are restricted for 45-footers, so you're SOL if that length or over.
If you legally want to drive a combination vehicle (think pick-up pulling a fifth wheel), and the trailer's over XXXXX lbs (sorry, the weight escapes me at the moment), you must obtain a non-commercial Class A (A = All). Maximum length for combination vehicles is 65 ft.
CA does share reciprocity with other states, so if you've got a 45-foot RV registered in AZ, SC, VT, MI, etc., you can legally drive it here with the appropriate license from your home state.
But you'd still be unable to venture onto those roads CalTrans has restricted.
Clear as mud?
FWIW & HTH. . .