Cirrus
Nimbostratus
I have personally only seen External Data Groups edging up to the 10K mark, but not exceeding it. I haven't heard of any limitation being reached but you might want to open up a case with F5 and see what they say (I highly suspect that they will tell you that any limitations will depend on the amount of load on the box that your putting it on rather than a Data Group content limit), then test the performance of the iRule.
Here is an article that lightly touches on the limitations: Writing iRules.
Creating data groups
When using the class command within an iRule, you can specify any of three types of data groups:
Addresses data group - A collection of IP addresses
String data group - A collection of strings, such as *.jpg
Integer data group - A collection of numeric values
The following sections describe these data group types.
Note: The size of a data group is limited by system resources only.
I would still suggest asking F5 directly for a definitive answer. (If they give you an answer other than what is listed in this article please share it with us. I'd certainly like to know what they say if it is something different.).
Hope this helps.