If everything on the external drive is in WMA format then that method would work, otherwise you are likely to get duplicates, which is why I suggested you work with the Not Added folder.
Different audio formats have different ways of compressing and storing the information from the original source. Most software and hardware will only work with a limited range of formats. iTunes cannot play WMA directly, but can convert it to one of the formats it will play. Given this is a clean install of iTunes that format is likely to be AAC if you haven't changed it. MP3 may be a better choice if you might want to put the media on some non-Apple device in future, otherwise ACC is supposedly slight more efficient, i.e. a smaller file for the same quality.
The main factor affecting file size is bit rate. The more original data that is kept, the larger the file and the more closely the reproduced music will match the original source. WMA is a lossy format typical bit rates are 128kbs and 256kbs. Conveting a 128kbs file to, for example, a 256k AAC file results in a file larger than the original, but no better quality.
If you put Windows Explorer in details view you should be able to enable a bitrate column and scan through to see what the current values are. You can adjust your import settings as given. You may want to import in more than one batch if there is a mix of bitrates.
tt2