Read the following Two pages linked below.
https://developer.valvesoftware.com/wiki/Net_maxfilesize
https://developer.valvesoftware.com/wiki/Sv_downloadurl
Basically if the map is larger than 64MB you will need Sv_downloadurl 1 and Sv_allowupload 1 in your server.cfg and also to point the clients to the URL / IP address containing the map'(s), so they can download it and the maps'(s) have to have the same folder structure as the server, (i.e., copy /cstrike/maps/... or /tf2/maps/... from the game server to /cstrike/maps/... or /tf2/maps/... on the HTTP server).
If the map size is less than 64MB than just using Net_maxfilesize 64 in the server.cfg will be suffice, however downloading a map from the server is not a very fast solution inc Comparison to the above solution.
Notes:
Bzip2 File Compression
Bzip2 is a program which can be used to significantly reduce the size of the files on the Fast Download server. Source natively supports Bzip2, so it's known that this will work. Bzip2 can reduce a file's size by up to 75%. This means a 50mb file can be 15mb or less, allowing players to download it faster. This also means if you have bandwidth limits, you'll save bandwidth with smaller files. Every single file that can be used by Fast Download can be Bzipped. However, the files on the Game Server must be the original files. Only the Web Server needs the bzipped files The normal files do not need to be on the web server if you use Bzip2.
Once the files have been uploaded to the Fast Download server (and put in the appropriate places), that's your work done. Source automates the rest, when the client downloads the file, they'll be automatically unzipped and cached etc.
You can either get Bzip2 and use the command line, or use 7Zip which has support for zipping into Bzip2 format.
Automation Utilities
SourceRSC: http://www.brainless.us/downloads.aspx?cid=1 can update your fast download server automatically. If custom content is missing from the fast download server it will bzip the files, create the appropriate folder structure, and properly 'virtualize' the addons and gamemodes folders on the fast download server. Supports accessing the game and fastdownload server locally or with FTP. Works on Windows and Linux.