'SAE' stands for Stamped Addressed Envelope. This means that you need to write your own address on the envelope and attach a stamp. You need to include this 'SAE' inside your letter, so that the people you are writing to can write back.

A business-sized envelope is called a DL envelope. This means an envelope which is 110 mm x 220 mm (or 11 cm x 22 cm). This is the perfect size to take an A4 sheet of paper folded into thirds.

So... a business-sized 'SAE' is a DL envelope containing your address and a stamp.