Take clones from the lowest part of the plant, they will root the fastest no matter how big or small they are. Taking clones from a flowering plant can take a bit of time to reveg depending on when you take them but clones root even faster when taken from a flowering plant. You can expect roots in as little as a couple days to 2 weeks. I don't bother using any clones that take too long to root as they can build up root rot that goes unnoticed.
When I first take a clone, I find one on the lowest part of the plant, cut it at a 45 degree angle, lightly shave a little extra at that cut then quickly dip it right into the rooting gel bottle.