There aren't restrictions on diameters any more, but cleat holding, longevity against minor chafe and kindness to crew should be driving considerations. Most professional rigging outfits can get you close on line lengths, or pull the existing halyards out of your mast and measure before ordering (replace with thin leader material so you can easily pull the new halyards).
Any thinner than 6mm or 1/4" for spinnaker halyard and the tail part of the genoa halyard become relatively difficult for crew to work with when heeled over and loaded up a bit. Most will opt for a tapered genoa halyard with a cover to add bulk and a good grip texture to the area to be cleated on the mast (make sure you have two 150 aluminum cam cleats for the genoa halyard, spaced an inch or two apart). Make the tail and the cleated portion of the genoa halyard another 2 ft longer than recommended and it'll give you room to cut off the splice and re-splice if you end up with chafe from the mast or headstay. Don't want to end up short.
The old rule main halyard diameter was way overkill, but you still want enough meat to pull the sail up and avoid having to replace the halyard every year. Any 6mm high tech is fine, double braid if you want, but most go straight dyneema.