City Ground is smaller (by a couple of hundred) than the Amex, as is Stadium MK. Bramall Lane and St Mary's are both about 2k larger. But see above for why I'm confident Brighton will host at least one England game.
Given that there are eight venues (Wembley only hosting the final) and four groups, presumably there will be two host venues for each group, ie three group games each.
Given geography, you'd expect the groups to be:
1. Brighton and Southampton.
2. Milton Keynes and Brentford.
3. Forest and...