Decorative pieces are measured according to SLOT/APERTURE/STRAP size, so if you buy a 20mm centre front decoration, then you would thread a 20mm strap through the slot. You can normally get away with the strap/fabric being a big wider on your bikini, as the fabric will simply gather through the hole.
Bra/Bikini/Dress/Bag Fittings - Trim - Opening Metal Buckle with diamanté's - 23mm inner diameter, Pewter Silver, each
Bra/Bikini/Dress/Bag Fittings - Trim - Plastic 'Square' Ring, 20/30mm internal, Vibrant ORANGE, each
Bra/Bikini/Dress/Bag Fittings - Trim - Plastic Jewel, 22x20mm wide (with inner slot 15x5mm), Freesia (purple), each