what size fittings

5/16 SAE flare. You can pull one of the fittings off the trans and then you'll have one to match

These are NOT JIC or AN, which is 37 1/2* flare, but rather 45* SAE flare

Most parts stores are not going to have steel flare nuts. You'll probably have to go someplace that makes hydraulic lines, etc, truck parts, or at least has a wider array of stock.