Have you ever done this then mate of either options?
I relocated a smaller battery in the boot and in all honesty, not worth the hassle. Bit of a ball ache to fit imho, running a cable though the bulkhead and under carpet, fitting a whopping fuse in line etc. That said, the car was doing my head in at the time! The battery died after not using it for 2 weeks, I bought a CTEK charger which saved it and did maintain it when it was on charge.
So another thing to consider if you do go for a smaller battery is the ability to keep it on charge, do you have a garage driveway where you can run a charger?
IF you did want to relocate the battery, then go for a normal size battery, that way you'll have no fear of it not starting after a few days.