There are a lot of different opinions and an abundance of different information out there regarding what generic fuel lines work best.
When in doubt, OEM lines and clamps are the way to go. They are a bit more $$ but they fit as they should and there's no drama.
The stock size appears to use a 7mm ID hose, which is more difficult to source. 1/4" ID hose is 6.35mm and 5/16" ID hose is 7.94mm. Either size is reported to work for different members of the forum. The proximity of the main and reserve fuel pick ups in the tank can make it tight getting both hoses attached all the way up. There are many types of tubing that will work, but it is preferable to use tubing that is specifically made for fuel. Standard hose available at the hardware store will also work, but might get hard and turn yellow over time.
I personally used thick reinforced black fuel line from Home Depot for over a year with no problem (1/4" ID, 1/2" OD), but recently swapped them out for yellow Tygon lines (1/4" ID, 3/8" OD). I used a little oil to lube the nipples on the tank and vacuum petcock and was able to get the nipples to fully insert into the lines. I reused my existing fuel filter. Since it was pretty tight, I didn't put any additional clamps on the lines. I didn't swap out the line from the petcock to the carb yet and so far only used a couple feet. 5 feet of tubing should be enough to swap out all of the lines.
I just replaced the three fuel hoses from the petcock on a 2006 GS500F. I couldn't source the OEM hoses. Actual measurement of existing stock hose using digital calipers was 6.5mm ID x 11mm OD. Not sure if the ends would be pre-stretched up to required size on the OEM hoses, as they need to be stretched up quite a bit! I used 6mm ID, 11mm OD hose with cotton braiding. You'll need a little over 1 metre to match the OEM length. I added 10cm or so to the tank hoses over the standard OEM length to give a little more play next time I need to lift the tank. I would use non braided in future as I had to carefully cut back the braiding which was glued/melted on to the hose in order to allow the hose to stretch wide enough to get onto the nipples/barbs. That took some time. I would also go with a larger internal diameter of 8mm (comment below suggests 10mm will also work) as suggested above as it was a helluva job getting them on to the 3x petcock and carb. Slightly smaller outside diameter would probably also make things easier. I softened the end in boiling water, used needlenose pliers to attempt to stretch the opening, and also a little grease on the barbs and even then it was very tough to get the hoses on.
"So I went and purchased some 8mm or 5/16" fuel line. Guess what! It fits the tank petcock perfectly, but doesn't fit the frame petcock!!! The frame petcock input lines are definitely a 10mm or 3/8" connection! Two 10mm lines only just fit next to each other on the tank petcock as the two exits are so tight together. They are a touch looser than I'd like but they should be fine if you clamp them on down hard!!!!" - http://gstwins.com/gsboard/index.php?topic=37366.0
Whilst I had the petcock off I also ground the tab off with a file to allow a 2nd main tank prime position for the main tank. Very quick mod allowing you to still have a reserve (petcock position to the front of the bike) whilst on the prime setting. http://gstwins.com/gsboard/index.php?topic=37366.0 .