DisCoverArt is based on the Google image search api. I doubt that you will find correct coverart using the artist and discogs_releaseid. Could be. You have to try it in Google.
If that works you have to check in MP3Tag if there is a parameter like %discogsid, then you could create an action using that parameter.
Perhaps you stored your discogsid in another id3-variable, then you have to use that below.
fi. for 300x300:
In the menu, select "Tools/Options/Tools" and click on the button "New" (button with yellow star).
Name: "DisCoverArt Discogs 300x300 Artist+Title NoQuotes Manual"
Path: select the path where you unzipped DisCoverArt.exe (f.i. C:\Program Files\Mp3tag) and select DisCoverArt.exe
Parameter: "%artist%" "%%" -discomusic.com 0 1 300 300 jpg 20 0
Do not Select the checkbox "For all selected files" and click "Ok".
ps. you can also first try manually in a command prompt using this syntax: example:
discoverart "" "" all 1 0 300 300 jpg 10 1