Probably worth mentioning:
There are two separate and distinct things when it comes to saving audio. We might be starting down the path to mistaking one for the other.
The first thing is "Replay". This feature gives the listener the ability to replay the last 30-240 seconds of transmissions. In other words it is an answer to "What did I just hear?" This is a rolling thing, meaning that it's recording only the x seconds and once that time is expired, the recording is overwritten with the next x seconds.
The amount of space taken on the SD card is scant and the length is programmed in the scanner and is not card size dependent.
The second thing is "Record". When this feature is activated, every transmission is recorded and saved to the SD card. I'd say this is more for a person who has to leave his scanner unattended for a period of time and wants to review what he missed.
(With Record being on, the scanner saves those files it created as part of the Replay process.)
SD card size becomes a factor with Recording. The more space the card has, the more recordings it can hold. A while back I estimated that each second of recording consumed about 20KB of card space. Recording happens only when carrier is received. Idle channels are not recorded.