There's no issue with different sizes - &, within the context of your drives & what you're likely to be doing, there's no issue with sizes & using the capacity.
Yeah, the only time that there would be an issue is if the number of times you're duplicating wouldn't work... ...so, imagining someone were looking at duplicating the entire pool, for example -
- with 2x duplication &, say a 1TB & 8TB drive, they could only actually duplicate 1TB
- & with 3x duplication &, say a 1TB, 4TB & 8TB drive, they could still only actually duplicate 1TB
...however, unless you're after >=6x duplication (which is highly unlikely), there's no problem whatsoever.
If you are using duplication & your current drives are pretty full already, after adding the new drive then I would suggest pushing the "Duplication Space Optimiser" to the top & forcing a rebalance run just before going to bed... As this then should prevent there being any issues moving forward.
