I am making a wedding cake to serve 150. I really didn't want to go higher than 4 tiers, including the top tier that won't be served. I am not sure what size tiers to use to get that many servings, but then not having the top tier be so much smaller than the others. If I did a 16/14/12, that would give me enough servings but then if the keepsake tier is only a 6 inch, would that look funny? Any suggestions on sizes to use? Thanks!

Going by the "Wilton Wedding Cake Baking Time and Batter Amounts" chart if you do round tiers you could do 8 inch round for the top tier (24 servings), 10 inch round (38 servings), 12 inch round (56 servings), and 14 inch round (78 servings) which would give you a total of 172 servings not including the top tier. The 8 inch may be a little big for a top tier to save and the total servings would be a bit higher than you need.

If you wanted to do square tiers you could do a 6 inch (18 servings), 8 inch (32 servings), 10 inch (50 servings), and (12 inch 72 servings) which would give you a total of 154 servings not including the top tier.

You could also use different shapes together, but obviously it depends on what you design is.

According to the Wilton chart, using the 12, 14, 16 that you said would give 234 servings, but you may be using a different servings chart. But I do think a 6 inch on top of that would look a bit odd. Here's the link to the Wilton chart in case you'd like to use it:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm