Battery size?

Batteris: There are only 3 companies left: Deka, Exide, and Johnson Controls.  I would only recommend Deka batteries.  Johnson Controls makes most of the Auto Parts store batteries, and they are usually lacking in quality. Even Optima have come down in quality since purchased by Johnson Controls.  Deka makes at least 2 versions of Group 24 battery, I had the higher output I was going to use in 1964 American Convertible, kept the battery when I sold it, and after sitting in my garage for 11 months, and the Carquest(made byJC) battery failing to start my Spirit, I dropped it in, and it started right away.

Also would recommend these products for cleaning and protecting the batteyr terminals:

http://www.grainger.com/product/CRC-Battery-Terminal-Cleaner-2F133?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/2F133_AS01?$smthumb$

http://www.grainger.com/product/CRC-Terminal-Protector-2F134?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/2F134_AS05?$smthumb$

Never hurts to upgrade the ground on any vehicle, I will be doing this to my Jeep Comanche very soon.

Can anyone tell me how the ground cables are in your cars? Mine goes from the battery to the block, then a second cable from the block to the front suspension? I am wondering it that's okay to leave it that way. I don't have any issues with anything not working, except the slow turn over at start up.

I'm fairly certain it goes to the top bolt holding the idler arm. If I need to move it, where on the cross member would it go?

The top bolt of the idler arm is fine. It just needs to be somewhere on the body, not the engine. You need a ground from the battery to the block for the starter. Then you need a cable from the block to the body for the rest of the electrical system. Anywhere below the motor mounts is fine. I've seen them on many different places. Some cars use a bare braided strap from and engine block motor mount bolt to a crossmember motor mount bolt, just jumping the rubber mount. Unless you're concerned about being 100% factory in appearance location doesn't matter as long as it's a good connection. 

It will be fine. The engine is mounted on rubber to stop vibration but also has the starter motor attached that needs high current. As electricity cannot pass through rubber and the rest of the car needs a ground as well there needs to be 2 points of attachment. I hope this makes sense. Just make sure the connections are clean and tight. Don't connect to something painted or covered in grease.