Expansion tank sizing

Can you help recommend a brand/size expansion tank for my HW system.

I currently have one but it is a large, old, steal, bladderless tank recessed in the ceiling. I need to remove it anyways cause I have to sheetrock around the boiler so I am deciding to replace it.

I have a 4 bedroom 2000 sf cape
Full basement
2 bath

http://www.watts.com/pdf/1915356.pdf

Nope. Need to know volume and head pressure.

Watts has a calculator on their website you can use. You'll need to know some stats for your heating system to get it right.

Do I only need to take into consideration the volume of my boiler or all the piping as well? If so, how do I estimate the 1' copper lengths throughout the house?

You need the total volume of water, since all of it can expand and contract. Volume of a cylinder is pi*r^2*length. If you get a guess at the total length of pipe, you'll be close. But, I assume those pipes run to radiators? Unless they are finned baseboard, they'll add some. If you get a good approximation, the tanks have a fair range, you should be able to properly select one. there are 231 cubic inches in a gallon.