Do you mean from the gasket face to the earth electrode? If so, I hope this is all lost in the earth electrode area, not thread length. The thread length is a common size, and only a few sizes exist, so engine manufacturers build to suit. If the plug is short, two problems occur. First, the firing tip may be recessed, reducing its effectiveness. Secondly, the unused threads get contaminated with carbon, making fitting correct plugs later difficult. I have seen this cause damage to the head, or seize a plug when short plugs have been in for some time, then replaced with correct longer ones.
Conversely, I've seen where a longer plug had been used, luckily not damaged valves or pistons, but the exposed thread in the cylinder got covered in carbon and brought the thread of the aluminium head out with it when removed.
This was all covered in some detail a few years ago when the Euro5 engine first hit the streets; I'll see if I can dig up the old thread from the 500 section.
Basicaly the Euro5/6 engines OEM plugs have a projected tip to place the spark a little deeper into the combustion chamber. The extra projection is 3mm and is all in the tip - the threaded part is exactly the same on both plugs, so no worries on that score.
It's all a bit of a joke, as the OEM plugs are so crude the spark jumps around all over the place - there's pictures showing that behaviour in the thread I referred to. The conclusion at the time was that the benefits of iridium plugs far outweighed any marginal gains from the extra tip projection of the OEM fitment and you'd do much better to fit the DCPR7EIX.
Some folks have tried the Denso equivalents, which IIRC are thought to be technically superior in terms of initial performance, but not so long lasting. There is also a Denso Tough plug, but these are quite expensive and my own view at the time was that the NGK Iridiums were the best price/performance compromise at around £26 (back in 2011) a set.
Long term experience has validated this and I am pleased to report no problems with either the Eu4 or Eu5 engines when running NGK DCPR7EIX plugs long term. As for durability/lifetime, the ones in the Panda have now done about 60,000 miles since fitting and both plugs and coil packs are still going strong.
If you do fit the OEM plugs (and I'd recommend DIY owners don't as the iridiums are well worth the extra money), there's absolutely no point at all in paying more to buy them from Fiat as the NGK part is exactly the same - all of them are made by NGK and this type of plug is only used in the Eu5/6 1.2 FIRE.
Why Fiat went to all the trouble of comissioning NGK to design a specific plug for the Eu5 1.2 FIRE and then specified it be made out of cheap rubbish materials is beyond me; it shows their penny pinching mentality at its worst.
If they'd specified a decent Ir fitment for the OEM plugs, list price of a franchised dealer 18000 mile service cost would likely be at least £100 less (and there'd be a lot fewer posts here about rough running engines with 10k on the clock). Copper core plugs are a relic from the days when plugs were gapped at 6k and discarded at 12k.
UPDATE: the original thread is
here; it gets interesting towards the end of page 1.