Hi,
I generally keep the LC Designer page size to match the printer page size. So here I would have an A4 page as the default LC Designer page size.
In Photoshop you can use a paper size preset for the new document. The actual size in pixels will then depend on the resolution you pick. So for A4:
3508x2480 pixels at 300 pixels/inch.
1754x1240 pixels at 150 pixels/inch.
If you open LC Designer and select the content area, go to the Layout tab. There you can see the dimensions of the content area (inches / mm / pixels). You can use this for your new Photoshop file.
Hope that helps,
Niall