OKX API compute for sz(size) "Order quantity must be a multiple of the lot size."

Question

https://www.okx.com/docs-v5/en/#order-book-trading-trade-post-place-order

In the documentation above I'm getting an error of:

"Order quantity must be a multiple of the lot size.".

Upon checking the documentation I have some questions about it.

1. In a BTC-USDT future trading pair what would be the "sz" in the documentation? Is it the BTC or USDT?
2. My USDT is 20 and my leverage is 10 do I need to pass only the 20 in the "sz" and assume that it will automatically set the leverage or do I need to compute the USDT and the leverage before passing it?
3. Upon checking the lot size all the future pairs in the OKX are in 1, imagine the minimum amount you can place in BTC pairs is 1btc since the lot price is 1.