Create a tileable square image from a pattern

Hi, been trying to work this out for hours and searched the forum but I can't find the answer.

Basically I have created a custom pattern and I want to save this as a tileable image of a precise size.

My problem is that I can't find out what size the pattern is in pixels.

My workflow is,

1) Open an image

2) In the Libraries window click on 'Add Elements'

3) Click on 'Extract from image'

4) Adjust scale,position etc. and click on 'Save to CC libraries'

I can then create a new file of 600*600 pixels at 150DPI and drag the pattern to it but the pattern doesn't fit the image exactly because the pattern is a different size.

Is there any way I can find out what the size of the pattern is or force the pattern to 600*600 so that it fits the image exactly?