The thickness of the layer will affect how well it supresses weeds, so it's hard to recommend a size. Generally the smaller particles will have less air space in between so theoretically they would block weeds better. However I'm guessing finer is more expensive due to the extra shredding involved. I have used a lot of mulch in those middle sizes and it works fine.

Google 'compost and mulch calculator' and you can put in your area and thickness and get cubic yards. Or, do it yourself: convert thickness in inches to feet (4"/12") and multiply by the area to get cubic feet. Divide that by 27 to get cubic yards. It comes out to about 7.5 cu yds. for a 4" layer.

"Tips for Carefree Landscapes: Over 500 Sure-Fire Ways to Beautify Your Yard and Garden" (a paperback book) says LARGE chunks prevent weeds better than SMALL chips.

One consideration is that large chunks will provide countless homes for slugs and small snails. Which have a tendency to destroy anything you plant in that bed.

While larger chunks do suppress weeds better, I use finely shredded bark mulch. It does a tolerable weed suppression job and also doesn't overmuch encourage the molluscs.

Air space between bark nuggets is important only by how much sunlight is allowed to pass through. Plants, even those unwanted ones we call "weeds" need sunlight to grow, so putting down a thick enough layer of mulch to block a plants access to that sunlight is requisite. Often,
I have found, larger nuggets have larger spacing and therefor require a thicker layer to do9 the same job as more finely ground mulches.

For 600 sq ft, 3 inches deep is 600 × 0.25 ÷ 27 = about 5.6 cubic yards, and 4 inches is about 7.4 cubic yards — so order around 7 yards if you want a full 4-inch layer, or 6 yards if 3 inches is enough and top up later. For weed suppression, go with the 3/4" to 1-1/2" size: it knits together and blocks light better than the big chunks, and lasts longer than the fine stuff. You can check the exact figure for your bed size on this mulch calculator.