that doesn't make you an idiot - au contraire, mon ami!  
With a P2PKH you have roughly 227 bytes per tx, but when you use the full public key (compressed or uncompressed), it get's a bit longer. A good summary was here: https://bitcoin.stackexchange.com/questions/1195/how-to-calculate-transaction-size-before-sending-legacy-non-segwit-p2pkh-p2sh
And yes, multisig and CSV/CLTV or any smart contracts increase the size. And then of course segwit, decreasing size :-)