Custom Caching Limits for OS X Server
Solution 1:
Your options aren't limited to the numbers printed below the scroll bar. You can drag between the options to select a value closer to your desired value:
You can set custom values if you really want to though. The cache size is stored in the CacheLimit key in the configuration plist which is located by default here:
/Library/Server/Caching/Config/Config.plist
<key>CacheLimit</key>
<real>268586281855</real>
The integer is bytes.