As I understand them, multisig transactions are actually transactions from multisig addresses (addresses requiring n sigs). Is this correct? (in other words, "multi-sig transactions" == "transactions from multisig addresses")
What is the limit on n? Couldn't an attacker spam the blockchain by creating a giant multisig address with large n? What are the precautions in place?
I assume im my answer that you're asking about P2SH Multisig transactions as you're mentioning multisig addresses (see What are the limits of m and n in m-of-n multisig addresses? for the 2 types of multisig schemes).
That is correct, usually what's called the multisig transaction is the one that spends from a multisig address. Note that prior to that, another transaction must have funded the multisig address.
The script length defines the limit. For each additional signature required the script will have to be longer and the limit in this case is 520 bytes. Basically, you can go up to 7-of-7 safely.