Group or fby

Hi Q Experts, 

I’m unsure of the best option in this instance so hopefully some of you can help. 

Say I’ve a table with columns date, sym and size (small sample);

date sym size

---------------------

2018.02.01 `A 200

2018.02.01 `B 400

2018.02.02 `A 600

2018.02.02 `B 200

2018.02.05 `A 100

2018.02.05 `B 101

2018.02.06 `A 205

2018.02.06 `B 400

How would I reduce this table to only show me, say 5 records per date (in this case, the 5 highest sizes for each date, obviously I can sort those columns first if needs be).

The big question - would I be better using group or would fby be more efficient here and also _ why _ more importantly? - I was reading into fby on the following page (but was finding it confusing);

https://code.kx.com/wiki/Reference/fby

As always thanks!

Regards

Andie C