I want to build a simple box with the top being a wire mesh and hook it up to a vacuum so I can sand over it and collect the dust. My question is how do I gauge how powerful a vacuum is? I don't want to use a regular vacuum cleaner for suction. I want powerful suction vac built in to the box. Anyone know what types of specifications I should look for when buying the vacuum? I have no idea how a vacuum is rated in terms of suction. Anyone know what specifications I should be looking for when I purchase the vacuum. Any tips on building the dust collector? Anyone ever build something similar?

Why don't you want to use a regular vacuum cleaner?
I know that ordinary household vacuum cleaners don't like dust from drilling/grinding concrete (small sharp particles) but sanding dust from styrene and resin should be OK.

I don't think you really need that much suction power to get the dust to move in the right direction, gravity is already working for you. It depends on the size of the suction box.
A suction area of 8 by 12 inches should be well within the suction power of a household vacuum cleaner (assuming the cleaner isn't full already).
The box should not be flat bottomed. Think inverted 4-sided pyramid with the suction hose attached in the center.
A circular box (inverted cone) would be better since there are no corners where there would be less suction.
A more advanced design could include baffles inside the box to distribute the airflow evenly.
/ Robin

Specs youd have to look up online. But if you want a powerful suction i would probably suggest getting a small handheld hoover for pets they are good & tough for picking up pet hairs of couches. As for the dust collect get an old stocking and tape it around the nozzle so it collects the dust but then you cant acidently drop your sprue piece and have it swallowed up by the monster. Another idea to that though is while the stockings over the nozzle get an old plastic bowl drill out the size of the vacum nozzle and then slide the bowl on the tube and stocking then you have a safeguard catcher incase you do drop the piece.
Hope this helps

I assume you're doing this because you can't wet-sand ?
What exactly are you sanding ? How big and what material ?

I am also in the process of building a sanding dust collector. I have a lot of resin kits and after market items and I am concerned about the dust particles becoming airborne. I could wear a mask while sanding but I have two dogs who keep me company and I don't want to affect them.
I have built a cabinet to house my shop vac. I did this for 3 reasons:
1) a flat work surface on top
2) some noise reduction
3) I put a furnace air filter in the door of the cabinet to filter the vac exhaust air
I am now constructing a work area to which I have attached the vac hose with screening across the front of the vac attachment. This will sit on top of the vac box/housing. I will also be mounting two 12" LED light strips for illumination. The dust collector is about 18" wide at the open end by 14" high and tapered down at the back to where the vac attachment is mounted.
I'll try and post some pictures.

And you’re not worried about small parts being sucked in? Sounds like a bad idea to me...

Get a 3 to 5 gallon shop-vac from Wal-Mart or home depot. More than enough suction unless you are using a belt sander.

I use a Tamiya or OLFA P-cutter to remove larger pour blocks as this removes a sliver of resin rather than dust creation.

I also have a sheet of wet & dry on the bottom of a flat tray filled with warm soapy water when needed, with a selection of wet & dry shapes.

Put a nylon stocking or cheesecloth over the hose, hold it in place with an elastic band. I used my shop vac last night to find a dropped headlight for my T-55, using cheesecloth to catch the part. Works great.

I have a piece of window screen in front of the hose attachment opening. For really small parts I will sand in the collector with the vacuum off. When finished I'll remove the part(s), turn on the vac.

A wet and dry vacuum should work it doesn't have a dust bag. We use it at work for the powersanders and for other dust from drilling in stone and wood. It is powerfull enough

There are companies that make attachments to turn a shop vac into a "wood workshop" vac system. They come in kits have all the pieces to hook up to pipes that attach to wood working tools (table saws, lathes, etc...) in a home workshop. Might be expensive, but you can use the pipes so you can run the vac in a separate room. Or, make your own options.
http://www.canadiantire.ca/en/pdp/shop-vac-workshop-sawdust-collection-system-0744410p.html#srp