It should be 64 digits (0-9 or A-F hexadecimal) or 32 bytes.
For instance, here's the "bitcoin is awesome" brain wallet in hexadecimal
23D4A09295BE678B21A5F1DCEAE1F634A69C1B41775F680EBF8165266471401B
To get it in WIF format, you can use an offline version of
https://www.bitaddress.org/Click download under the donation link, put it on a thumbdrive, take it to a computer with no internet access. (Preferably one where internet access can be hardware disabled.) Unzip it, double click bitaddress.org.html.
Click "Wallet Details". In "Enter Private key", type your digits. Click "View Details". It will give you a public and private key in various formats. Go to a different computer with online access. (Or use your phone) and verify the public address given has the balance you are expecting.
In the case of "bitcoin is awesome", its public address (listed under the "Bitcoin Address" and a QR code on the left) is: 14NWDXkQwcGN1Pd9fboL8npVynD5SfyJAE
You can append the public address to this url:
https://blockchain.info/address/Like this:
https://blockchain.info/address/14NWDXkQwcGN1Pd9fboL8npVynD5SfyJAETo check its transactions. If it is the proper wallet with the balance you expect, write down the private key from it in WIF format from bitaddress.org. For "bitcoin is awesome" it's: 5J64pq77XjeacCezwmAr2V1s7snvvJkuAz8sENxw7xCkikceV6e