Hi Greg,
You are right, by default the maximum number of items that can be displayed in a dropdown list is 50. The reason behind this is page load time. For example, if you had a list of call numbers and you wanted to get to the items containing '70' you would start to type and items containing those numbers will filter to the top as you type.
In essence, as you type, each key stroke makes a ‘call’ to the Method servers and quickly sends info back displaying in this case the top 50 matches. As a result, a dropdown displaying hundreds of items would slow down the dropdown response time.
Just out of curiosity, why do you need to display more than 50 items?