The blokarts have 6" wheels on the rear and 12 1/2" x 2 1/4" (or is it the other way round) on the front with the option of fitting the "Big Fork" and having the same wheels all round.
Many of the blokart racers (including Paul Beckett) are using that configureation.
The X-Sail looks to have that configureation as standard.
The blokart branded rear wheels seem to be made of a harder and more brittle plastic than the earlier "off the shelf" wheels they were using. I have busted two of them when the rim clipped the concrete edge of a path I was sailing on and went onto the grass.
They are precision manufactured though and have good balancing. It is hard to find tyres that balance well. Blokart use the "Deli" tyres which are not bad on balance but are only 2 ply, can be worn out in a session on tar seal or concrete and puncture quite easily (dry grass stalks are their enemy

)
The "Deestone" tyres have stronger construction (4 ply rated I think) and tougher compound but balance in them is poor which is the enemy of speed.
All round I think the Fallshaw 8" wheels are the best for a mini yacht in Australia.
From what I have seen of them the 8" wheels on the Seagull and other European yachts would be the highest quality of all. In Australia I believe they can be bought at shops that sell kite buggies but expect to pay more for them than the Fallshaws.