Is there a length maximum on how far your battery switch can be from your batteries?

Short of redoing my entire back seat to make the hatch more accessible, the battery switch needs to be relocated. I would like to have it mounted on the center console at the floor level by the drivers footwell..

It would take the switch about 7' from the batteries..

Any problem with this?

You can make them as long as you want as long as you resize them as needed for the run.

What size cable do I need to run for 7 or 8 feet distance? Two batteries running 300 Promaxes..

at least 2 gauge

Use the wiring size charts to determine wire size needed given both load and distance variables. Also keep in mind most all charts are 1 way lengths (battery to item). DC wire sizing requires the full wire length to properly size wires, meaning you add the length of both pos and neg wire lengths. Also factor in voltage drop %. Rule of thumb is max 3% drop for critical items, ie bilge pumps, and 10% max for non essential items such as courtesy lights.

I always use 3% max for everything though. I figure going to provide power, give it all it needs. I have 1/0 feeding my stereo amps. No loss of voltage there, but going 20' away (40' total wire length)
If they were mounted right next to the batteries, I could use speaker wire to power them. Wire length is the killer for DC loads.

7' is no problem. I would run at least 1/0 gauge wire.

1/0 for a pair of eggbeaters??? Its your money

Don't forget those cables have to run threw the cowlings.