At a guess, 1/4" ID should be fine. My F115 uses 3/8" ID. If you already have the fuel connectors, simply measure the barbs. If you don't, get them first, as they'll probably determine your hose size.
8mm (5/16) is a pretty good idea and will suit any engine. Use decent SS hose clips. As above I also added an additional inline filter by the tank end, only a few quid each on eBay and easy to change each year.
This is the hose I got below to re-do my lines, the Mercury original hose is poor quality and has a plastic liner that is a PITA when cutting as it splits and blocks up the bore when pushing in fittings etc.
