One option is to create a custom dropdown editor that has a search. Not sure if anyone has created this already, but here is some documentation on how to create a custom property editor:
Another option is to create two or more dropdowns to filter down the results. This blog post explains how to select a region first and then a country within that region:
We have a requirement in our website where we need to select a country name from long list of options and we added dropdownlist property of episerver to implement that, but we can't type in dropdown to select from dropdown we have to scroll down all to select one. Is it any other approach or property we can use to achive this ?