Go here:
http://oakland.athletics.mlb.com …allpark/seating_chart.jsp
Print it out enlarged. Mark a piece of paper with one mark on home plate and one on first base. Label te first mark "0" and the second mark "90". Make 4 such marks equally spaced at 90 foot increments. Subdivide each tick mark in half. Write the distances by each tick mark. These would be 45 foot marks. You'll have a ruler marked like this:
0-45-90-135-180-225-270-315-360
Now find your seat and put the first mark on your seat, and you can "measure" distances to all parts of the ballpark.
The assumption is the seating chart is drawn to scale, but there's no reason to think it isn't.
Here's what the view from your seat will look like:
http://oakland.athletics.mlb.com/oak/ballpark/seats_3d.jsp
OK, now that's you've done all this work let me burst your bubble: Unless you're sitting in the first couple rows around the infield, ANY lens you take will be too short. You can't possibly bring in a lens that will be too long.
Here's what the A's site says:
Cameras and Videos:
Cameras and videos are allowed in the Stadium during all regular season baseball games as long as they are not for commercial use. Both cameras and videos must be hand held; tripods cannot be set up in the seating areas or the aisles (monopods are allowed), and must not block the view of the ball game for other guests.
Carry-in Items:
All bags must be no larger than 16" x16" x 8". All bags, including backpacks and purses, are subject to search. No cans, weapons, glass containers or alcohol are allowed into the Stadium at any time. No hard-sided coolers of any size are permitted. Diaper bags are permitted in regardless of size, for guests accompanied by their babies. Strollers must be small enough to keep out of walkways and aisles or be checked into the Security Office 125 or First Aid 109.
So, as in many parks, if it'll fit in the 16x16x8 pack, and you follow the rules above, you're golden. Plus, contrary to many places, you can use the monopod on your EF 400 f/2.8L DO (the only 400 f/2.8 that will fit in this size bag). A 300 f/2.8 should fit in there, though.