I believe that the average words per page for a paperback book is 350 words. So, to figure out how many pages your manuscript would be as a paperback book, you would have to divide the word count of your novel by 350.
Here is an example. Say your manuscript is 89,557 words long. You will divide 89,557 by 350. The number you get from this division, 256, will represent an estimated page count of your novel as a paperback book. Your actual page count will likely be more because novels have unused portions of pages due to chapter breaks.
In order to estimate how many paperback pages your first chapter (10,000 words) would be, you would then take 10,000 and divide that number by 350. You will then find out that your first chapter would be roughly 29 paperback pages.
For your optional question, most will tell you that there is no correct length for a first chapter. Your first chapter will be as long as it needs to be. It could be one word, thirty, fifteen hundred, or twenty-six thousand. What really matters is whether or not your first chapter is gripping, catches the readers eye and is well written. My advice is to ask yourself these following question. Does the first chapter tell a basic story? Does it have a beginning, middle, and end? Is there conflict? Is this conflict resolved or is there more to show? Does the first chapter tempt further reading of the novel?