If your list of possible choices grows and grows I'd guess that there is quite likely the possibility that no matter what value you put into the DisplayCount property, the users may still need to scroll down.
One possibility is that if you are using an ALIAS as the RowSource for your ComboBox, you could use a table/cursor which was ordered/indexed by the number of occurrences for any specific entry.
In that way, the most frequently used selections would always be towards the top thereby minimizing the need to users to scroll down.
Good Luck,
JRB-Bldr