I used 1/2" Acme for X & Y and 1/2" ID bearings work fine. I bought at this guy's eBay store - you can get different quantities - 10 for $7 is a good deal. They're not flange bearings. I used a 1 1/8" forstner bit to drill a .3" deep hole for them to sit in on the gantry side and the front piece and put two acme nuts on the screw up against them. The other side of the rods is up tight against the motor in 1/4" to 1/2" Lovejoy couplers from Enco.
I used a flange bearing as called out in the JGRO plans for the Z axis on 1/4" Stainless rod, but I had to put the flange bearing on the top of the bottom plate. Since I used a Lovejoy coupler on the Z motor, it would come apart and the Z carriage would just fall down if the flange is on the bottom side as shown in the plans.
Here's a picture of the Z axis - you can see the coupler at top and the flange bearing on the bottom. There's a black washer and a double nut on top of it.