26T in my experience. At that time it was the largest cassette...the 13-29 was for the Racing T only. However I have recently fitted a mech dropper to my 1996 Chorus rear mech and 27T runs very nicely - it used to be a bit rattly and the teeth were definitely almost touching. I reckon 28 should be fine, 30T possibly.
The smallest that fits on the front is 39T on a Campag chainset. I have run a Record front double with 44/29 on my MTB. The smallest front size possible is most dependent on the difference between chain ring size...15 teeth is definitely doable but also depends on your frame. For example will the tail of the mech clear the chainstays, which in turn depends on seat tube angle.
