User data (flight plans, custom waypoints, etc.) take up a negligible amount of space. Also, unless you're puling SD cards from a museum, the base iFly software will probably fit on the smallest card you can find with room to spare.
The primary size driver for your card is going to be how much of the available aviation data you want to download. There's a big size difference between carrying around just the VFR data for Arkansas or Idaho vs. all the VFR+IFR data for the entire USA.
You mentioned that you have backup cards (plural). You could just use one of those and be done with it. But if you want to replace the card you lost and want a guide to what size you need, pop one of those backup cards into your PC, right-click on its drive letter in Windows Explorer, and select "Properties" to see how much space is being consumed on it. (If you want to see the maximum space iFly can consume with data, then update one of those cards with the entire USA dataset and then check its Properties.)
If I recall correctly, the SD card must be formatted in FAT32, which has a maximium capacity limitation of 32GB, so any card bigger than that will just have unusable/wasted space on it. If you're buying something new, 32GB cards are easy to find for under $10 so there's little financial benefit to trying to get anything smaller to save money. If you're scrounging around in the miscellaneous computer parts shoebox in your closet, just look for something big enough to hold at least what you see saved on your other backup cards.