I just noticed you post, so I hope I'm not too late to help you.
As for the question about length; I've seen an Mi-8 and an Mi-17 parked together. if there isa difference in length, it's very small and not discernable with the naked eye on the ground.
The Mi-17 has the older style clamshell cargo doors while the 171 has the newer ramp at the back.
Outside of the clamshell/ramp style cargo door; the biggest differences between the 17 and 171 are seen on the right side of the fuselage and the tail boom.
Here's an Mi-17 (sorry for the poor scan quality):
And an Mi-171:
The most noticable differences are:
The movement of the oil cooler from the front end of the right side sponson to a spot further up on the fuselage side, between the rotor and the side cabin door.
Equipment housing moved further aft along underside of tail boom.
As for the cockpit:
Here's the Mi-17:
And the Mi-171:
Outside of the switch from the classic, lurid Soviet turquois colour; some of the older analog instruments have been replaced by digital counterparts.
Additionally, the instrument faces and placards in the Mi-17 were all in Cyrilic, while the Mi-171 has them all in English.
You did make mention of of an avionics compartment on the nose of the Mi-171; which I'm assuming you were refering to a blister like fairing of the sort on this Croatian example:
I've been told it houses everything from a weather radar to a GPS antenna. In the cockpit, the only difference seems to be the presence of a small general purpose screen bolted to the upper right corner of the left side main instrument panel and another indentical one bolted to the upper left corner of the right side main panel.
It is an option that is not seen on all Mi171s, as evidenced by the Czech machines:
That covers the main differences.
I do have quite a few more detail pictures of Czech Mi-171s that I can post if you'd like me to. Just say the word.