How do i change presentation slide to size A1, whats the equivalent in pts.

What is it your trying to do that you want to use an A1 size?

Keynote can print either; handouts, slide with or without notes or the text outline, when printing these you make appropriate settings for paper size in the print driver for the printer you are using.

If you want to use Keynote for page layout, don't, Keynote can't scale to paper sizes. Use Apple Pages instead.

A1 paper is 594mm x 841mm, so you can use those dimensions as pixels, or better yet a multiple of them. 4x gives a nice size:

2,376 x 3,364. If that is too large, 2x is 1188x1682.

Go to "Slide Size" and choose "Custom Slide Size" and enter the numbers.