I have a potential wedding cake to make and they are needing 250 servings. What size tiers could I use to get that? They are wanting round tiers and I'm sure it would need at least 4 tiers.
Also wondering if the largest tier would even fit into a standard size oven?

http://www.wilton.com/wedding/wedding-cakes/wedding-cake-data.cfm

Well, you're going to be hard pressed to get that many servings out of a four tier round. A 16, 12, 10, 6= 200 servings (Wilton)

Might want to look into a grooms cake, or sheet cakes for the extra servings?

The below link is for the Wilton wedding cake standard serving guide. I live by this thing:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

Personally, I would make a 16" (100 servings), 14" (78 sevings), 10" (38 servings), 8" (24 servings) and 6" (12 servings). The total servings is 252.

If you can bake an 18" round or square cake you have these options.
you could do round 6" 10" 14" 18" = 260 servings.
or round 6" 9" 12" on a 18" sq = 260 servings.