Hello Cap,
It is a question posted last year, but it might be useful for others looking to fix this problem, as I have the same
It took me a lot of time to figure it out, but following Stack Overflow
Seems is impossible to do, as it is the browser deciding the dimension of this drop-down suggestion if you are using the data list function
The <datalist> element is very inflexible - you cannot style it at all as its styling comes from the browser and the browser only.