Now, unless you've got a model railroad background, the scales and measurements on that page will probably have you scratching your head in confusion. The first time I saw the page I had no clue what I was looking at. Particularly how to convert the scale and real-world size of the item into something useable.
"Just how big is a 1/4" scale 1" square nut with bolt?"
"Oh. 0.021 inches!"
"Wait... How big is that"
I went with the trial-and-error approach with my first order. I got a smattering of sizes and here's what I've found - when ordering, look at the shape to figure out what you want, then look at the column labelled "Meas. (inches)" to figure out if they've got it in a size that'll work for you. Rivet #154, 0.043", is approximately the same size as the rivets on most GW kits, and the one I've been using on all my Iron Warriors conversions. The smaller bitz tend to look real small real quick and are only good for the fiddly-est pieces of kit - engines, infantry equipment, etc. For vehicles, stick to anything 0.043" and bigger.
I don't have any of the tinier bits handy, but here are a few of the Augmentables from my recent order:
As you can see, they come 20-35 rivets/nuts/bolts per sprue and there are 5 sprues in each $3.00 pack. Coupled with their ease of use, they really can't be beat. Speaking of ease of use, I snapped some quick shots of how I use them.
Materials: model, glue, rivets, drill w/ 1/32" drill bit, hobby knife
Step 1: drill holes where you want rivets
Step 2: cut some rivets off the sprue
Step 3: put a tiny dab of glue over a few holes
(use a thin glue to avoid "glue bubbles" around the rivet)
Step 4: use sharp edge of hobby knife to place rivets in holes
Step 5: position the rivet flush with no drill hole showing
Step 6: fill all your holes
Step 7: use up all your rivets
Step 8: order more rivets
They're stupidly easy to implement, more consistent than any "home-made" rivets I've ever tried, and having them drilled into the model makes them far less likely to fall off your model from rough handling than any of the "glue to the model's surface" rivets I've used. For Orkz, I use a variety of sizes and shapes, usually switching up the spacing and rivet type between armour plates to give it that haphazard, sons of Gork & Mork feel. For Imperials, I find the regular rivets work wonders and try to keep consistent spacing throughout to give it that mass-manufactured look.
Hope some of you find this useful, and happy riveting!