I've owned maybe two dozen HI Khuks, maybe more, of various sizes (12" Sirupati smallest, 25" Kobra the largest, or maybe the 18" AK).
Anyhoo, I think the best "all around size" are the 15-16.5 inchers. I love my 15" BAS and 16.5" WWII.
For larger tasks or when you don't mind looking a bit scary, the 18-inchers are probably the best all-around "work horses".
For even larger tasks and when you don't mind scaring the snot out of someone, the 20-plus inchers are amazing. I cut down a fair sized sapling with a single leisurely stroke of my 21" Chitlangi.
For that matter, a rather leisurely stroke with my 15" British Army Service split an 18" long 2x6".
Note my use of the adjective "leisurely". Man, I'm not winding up and powering through these at all - the Khuk does all the work - and man does it ever.
If you want pure raw chopping power, the AK's are king with the WWII's right behind.
If you like a little more versatility and are willing to sacrifice the cutting ability a wee bit, the BAS, Gelbus, Chitlangis and Sirupatis are wonderful.
My favorites are:
15" BAS
16.5 WWII
18" Gelbu Special
Tie between 21" Gelbu and 21" Chitlangi (The Gelbu feels nicer in hand, but the Chitlangi seems to have more power in the cut)
Don