Is there a size limit or recommended maximum size for external datagroup files.

We use an external datagroup file with over 4000 entries that's checked against by an iRule a hundred times per second, and we've seen no issues. CPU never goes above 15%. BIG-IP 6900 appliance is what we're using.

There is no defined maximum size for datagroups so capacity is actually only limited to system resources. On newer platforms it's possible to store up to a few million records. As for performance, the lookup values are hashed and stored in memory, so searching is reasonably fast. If you want exact numbers, your best bet is probably to set up timing and look at it under test.

https://devcentral.f5.com/wiki/iRules.timing.ashx

Has anyone evaluated doing large data group re-direct at f-5 level vs. Akamai level? Any thoughts on management and performance?

If you have that many redirects you may want to look for patterns. That might save a lot of work/maintenance.

Recently our content group sent me a list of around 15,000 redirects. I was able to bring that down to maybe half a dozen pattern-based redirects and perhaps a few hundred unique redirects.