in the new server; data drain any LARGE sized monster.

To obtain Virus Core F, you must perform Data Drain with large-sized monster on the field with level 31 or more. Please note that you won't get Virus Cores all the time, may be you'll get items instead. Check the corresponding battle level at the right of your screen when you are going to warp from Chaos Gate.
Information you may want to know:

Battle level: <= 30
Small-sized monster: Virus Core A.
Medium-sized monster: Virus Core B.
Large-sized monster: Virus Core C.

Battle level: > 30
Small-sized monster: Virus Core D.
Medium-sized monster: Virus Core E.
Large-sized monster: Virus Core F.

I think he's asking where you can find any large-sized monster, yea I have the same problem too I can't find any large monsters in the new server with field levels of 31 or above

What level your at?

The easiest example of large sized monster is "Thousand Trees" or something like that.

Helba gives you an email on this...

I found one at a level 34 earth area