How much space does a backup of a wallet usually take? I have a Bitcoin QT v0.8.5-beta wallet (downloaded in 2013) which I will backup before trying to export the private keys to a Electrum wallet. I have found a backup button in the QT wallet, but it doesn't describe what the backup leads to.

I have downloaded about 1/15 of the blockchain (224 weeks behind) in the QT wallet, and I have a pendrive with about 2 GB free space that I intend make a backup to. So should there be enough space on the pendrive to take the backup?
Wallet backups are a few megabytes, depending on the number of transactions it has made.