Legal Question in Consumer Law in California

There is no 'legal font size" in general, though of course anything must be legible to have legal effect. There are legal font size requirements for certain specific provisions in certain industries and transactions, but without knowing your industry and the transactions involved there is no way to go into those here. You should also bear in mind, however, that terms and conditions on an invoice issued after a transaction is consumated and never signed by the other party will be very hard to enforce. There is a lot of law about "battle of the forms" in Uniform Commercial Code jurisprudence, such as when the customer issues a P.O. with pre-printed terms, and then the merchant issues an invoice with different terms. But unless your terms are accepted expressly or by implication before or at the time the contract between you and your customer is formed, they will not be enforceable.