I hope all is well,
Having looked into this, we can't actually change the width of the filter drop down, as this is the default 'set' size that is used within Yellowfin.
I checked the Filter Formatting options and the only options that it gives you with regards to 'width' was Default, Medium and Long, although this only made changes to the filter box itself.
See images below:
Going forward I can put this forward as a Enhancement Request if this is the direction that you would like to take this, although I can't guarantee an ETA.
I await your reply.
Here is the long format example, and notice my list item are still cut off.
Is there a way to change the CSS that would increase the width, or enable horizontal scrollbar?
Unfortunately this option is not available within the CSS side, so going forward the only solution to this is for me to continue and go ahead and put this forward as an Enhancement Request.
With this I can't give an exact ETA although this will be passed on to the Dev Team so they can investigate further.
I do hope this helps.