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.
Just a quick note, there is a difference between the wallet and the blockchain. When you back up a wallet, you are backing up your private keys, addresses, etc., not the entire blockchain. The blockchain takes around 150GB as of writing this post (see here: https://blockchain.info/charts/blocks-size), but a wallet backup is much smaller, as eponymous said.
You can back up the blockchain too, it is found inside your bitcoin data directory, but that is unnecessary because it can always be redownload from the network if needed. Only the wallet itself holds your keys which you must back up to keep safe.
