I'm writing a 2d particle engine using OpenGL, and I use points, lines and point sprites to represent particles. I'm using glBegin()-glEnd() way of drawing. I'd like to use glDrawArrays(), but I have a problem with that : every particle has it's own size, but glDrawArrays() doesn't allow me to draw primitives/point sprites of different sizes, as far as I know. Is there any way I can do this?

Thanks in advance

It looks like there is no standard way to set point size in array for glDrawArrays.
Also I don't think this would give serious performance boost since particle data changes frequently (every frame).

I think you should be able to do same thing by writing simple shader (GLSL (OpenGL 2.0), or any other). In this case, though, you'll have to use triangles to build sprites and textures to make points look round. Shader will deform triangles according to point size and texture coordinates of vertex. In this case you will be able to feed per-vertex data using glVertexAttribPointer (OpenGL 2.0) or any similar function available through extensions.

If that is too much for you, then just stick to glBegin-glEnd method, until particle system will produce serious slowdown.

Somehow it seems to me that I the small boost I will get from using the array method will be lost if I use shaders and triangles. Well, I'll stick to this for now. It's going to be an open source project (when I make it useable and embed Lua), so someone will hopefully improve on my code.

Well, it won't be lost (you'll need just vertex shader, not pixel), but this system will need a lot of redundant data to work.
I implemented such thing on DirecX many years ago when I've run into troubles with pointsprites (maximum size limit, no rotation, etc) in 3D particle system.
Implementing what I've described makes sense only if you need large quantites (over 1000) of really large (possibly larger than screen), rotating, perspective-corrected textured sprites acting as particles. If you are using points and they are sufficient for your task, then writing this will be (probably) a waste of time.