Pen pals - medium to long letters

How is the size of the writing paper? is it A5 or A4 size?

Either. Any reasonable-sized sheet. Maybe add an extra one if you're using those long, narrow refrigerator magnet pads. If you start with a notecard, end with an extra sheet or two. Some people write big, small, or double-spaced. Try to be reasonable and make it equivalent to two sheets of letter-writing paper.

I'm sorry if what I'm going to write is upsetting to anybody but I don't call a 2 pages letter a medium or long letter, for me it's just little notes and I hate receiving such very short letters, it's so disappointing and I decided to join only swaps with a minimum 4 pages, or more. I needed to tell this. 2 pages isn't enough to know somebody I think...

The 2-page minimum is for the intro letter. You can write more. For the reply, you can write as much as you like. Personally, I sometimes find it difficult to write longer for the first letter, particularly if there isn't much to go on from the partner's profile. You are welcome to write a wonderfully long letter of whatever length you wish, as long as the first letter is at least 2 pages.

The problem isn't what I could write but what I would receive. I know by experience that when the swap requirement is to write a certain number of pages minimum, people usually don't write more. I've been disappointed several time and for this reason I won't join this swap but I understand what you mean.

I'd encourage you to host your own swap with your own rules. Then you can be sure to have the exact experience you would like. Thank you for your interest in this swap. I hope the participants here will have a good experience.