What model/year car? That would help.
Also, as regards a battery being "too big", that can almost never be the case. The car's gonna ask for a certain amount of juice, and the battery's going to produce it. Say, your car needs 500 amps to crank it...even with a 1,000 amp battery, the starter is only going to get what it asks for.
I was in a similar situation with my 300TE. I replaced the battery box with the part from the 300TD (diesel) and it popped right in. I then installed a Die-Hard Gold with I believe over 1,000 amps and it's been wonderful. My wife left the interior lights on just last night, ALL night, and the car fired up this a.m. no sweat.
I've stuck with Die-hards, as they've lived up to their name. However, I've got a big Interstate in my 500E (likely the same battery as my 300 now has), and it seems great and is well-respected.
Best regards, Michael