For personal reasons I would like to know what pay I am expected to
get in the coming years. Is this something I should be able to ask of
I would like to know a rough estimate of what I could be earning next
year and in five years time etc.
Well, they say "there are no stupid questions". So from that point of view you can ask anything.
But I'd be shocked if a tiny company would commit to, or even comment on, future years' salaries in anything other than extremely general terms. In my experience, five years is an eternity for this size company, and nobody would try to predict salaries that far out.
You might hear something like "We tend to pay market rates." I wouldn't expect much more than that. Of course who knows what the market rates might be in five years?
I've worked at a good handful of small startup companies. If you came to me with that question, I'd say something like "Well, that depends on lots of factors - some of which are in your control, some of which are in my control, and many of which neither of us can control."
Then I'd point out that you should work as hard as you can in order to give you the best chance at the best salary and that this was pretty much the only part you could control.
In larger companies, I might review the company's formal process for determining and granting raises. I'd describe the meetings, the reviews, the levels of approval, etc.
But I'd never give you numbers, that would be foolish. And in most cases, since the numbers are derived from annual budgets and budgets are seldom set up far ahead of time, I couldn't know the numbers anyway.
So if you are looking for hard and fast numbers, I'd suggest that you not waste your time.
If so how should I do it?
Just ask. Something like "For personal reasons I would like to know what pay I am expected to get in the coming years. Can we talk about it?" should start things off.
Note: I can only speak from the point of view of every US-based small company I've ever worked at. You haven't indicated your locale - perhaps things are different there.