It's a very small download, even with extra instrument packs. Total download (on Linux) is just 44 MB plus about 10 MB more for the free extras ( MB not GB !).
The reason for this is simply that sound is synthesized using physics (maths) and not sound samples (which is where those sampled instruments are a pain).
All the instruments are included in the application as well, it's just that they are locked until you purchase an unlock code. You can try them all in demo mode along side the instruments you actually purchase.
All in a tiny download - the last version I downloaded was just 44 MB ! Yes, that's "megabyte" not "gigabyte".
Also remember to download the great free instruments when you have your purchase - that's another 9 MB for the lovely KViR historic pianos and keyboards, and another 1 MB for the bells package.
If you ever purchase new instruments (and you probably will - it's hard to resist) they are unlocked - no large download needed for those either.
You can register your new Pianoteq version using either email or online, and it's a pretty simple operation that does not require any special hi-speed internet connection. Once registered, you're done unless you want to download updates (which so far have been consistently appearing from the Modartt team). If you have any problem with registration you'll find that Modartt are usually very helpful in sorting out any problems - I think most users (like me) have been impressed by the support as much as by the product itself.
Note that if you are considering versions, the Stage, Standard and Pro versions all produce the same sound and use the same instrument presets. What differs is the level of customization you can achieve. With Standard you can adjust microphones and tune extensively and unless you have very, very specific sound requirements there is no need to get Pro (which does have more detailed sound customization, but you really need to have a good idea what you're doing before diving in). I'd recommend Standard and then upgrade to Pro later if you feel the need, unless you are sure you need Pro. Stage lacks microphone placement support and tuning options and I recommend those, as well as some of the other options Stage supports over Standard. I started with Stage and upgraded later to Standard and while I love tweaking sound, there's loads of options in Standard and I just lack the technical knowledge (and ear !) to exploit Pro properly anyway.
Incidentally Pianoteq Standard and Pro can use impulse reverb files (but have decent internal reverb options as well - I particularly like the new Piano Room 1 & 2 options), and the impulse reverb files I use are a 500 MB download compared with Pianoteq's mere 44 MB. Pianoteq is tiny !
StephenG