What size seat pack do I need?

I have a 50cu in Trek Pro Pack (under seat pack). Its stuffed full with the following:
1 spare tube
very small multi tool
3 small tyre irons
small patch kit

with that in there, I can just cram in my cell phone and a pack or two of gel, no way I could get a few power bars or a second tube in there. This thread had me curious so I had to look up on the Trek site to see which one I had.

Hope this helps..🙂

BTW, I thought the TREK PRO PACK bag was pretty cool til I got caught in the rain, does not seem like these are very water proof.. I had to open it up an let it dry out for a day or two - I'm not sure if they make ones that are water proof, but I'm a newbie so just something to consider when picking one.

I also posted the exact same original question on bikeforums.net, forums.roadbikereview.com, and the New York Cycle Club message board.

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The New York Cycle Club responses were incredibly helpful. Among the suggestions were
- Jandd Mountain Wedge Expandable
http://www.jandd.com/detail.asp?PRODUCT_ID=FMWEX

- Topeak AeroWedge
http://www.topeak.com/2006/products/bags/aerowedgepacks.php

- Avenir BigMouth 100
http://www.nashbar.com/profile.cfm?category=&subcategory=&brand=&sku=16678&storetype=&estoreid=&pagename=

- Cannondale FastBag 240
http://www.performancebike.com/shop/profile.cfm?SKU=20925&subcategory_ID=2310

- TransIt Macro Wedge
http://www.performancebike.com/shop/profile.cfm?SKU=14684&subcategory_ID=2310

This morning I bought the Topeak AeroWedge, but my wife fell in love with it for her bike & confiscated it immediately! So I ordered the Avenir Big Mouth 100 for me.

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In contrast, the responses over on forums.roadbikereview.com & bikeforums.net consisted almost entirely of folks telling me I *didn't* want a seat pack that big, and that I should be packing most of the stuff I listed in my jersey pockets, and that "no one needs two tubes" (or, "no one needs two botles of water, so use an empty bottle to store extra tubes, PowerBars, or your cell phone"😉. So bikeforums & roadbikereview can kiss my chamois; I got what I wanted & I'm happy, no thanks to them.