About 2-3/8 x 7/8" not counting the a full length guide along one 7/8" side (which you wouldn't need). And maybe 5" long. You might just look for a junker magazine to use instead. Or if you have Sportman's Warehouse nearby look for one of their $3.99 20rd Thermolds or add one to an online order.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek