What's the best size to make a brochure in Photoshop ?

There are no so-called best size for the project Data must put all clear here and choose the size that is appropriate for this Data

It depends, how much your contents, and how you with divide it to sections.

You can mack it a A5 brochure with1 fold, that gives you4 sides  ( x cm). this is the common size.

or A4 brochure with2 folds (Trifold), that gives you more sections.

check this link: 

http://www.webanddesigners.com/how-to-create-a-print-ready-brochure-design-in-photoshop/

I think you should go with the standard by8,5 inches document. and its better if you set the resolution to pixels per inch.

1. Dimensions (WxH):×8.5 inches (a landscape oriented letter sized document)
2. Resolution:ppi (minimum recommended for printing)
3. Color Mode: CMYK (for four color printers)

Dude there no such thing as best size it depends on how you will propose the brochure base on the content, client request or design. Just make sure your content font are readable enough.

it doesn't matter to use PS, ID or AI.

and size also does not matter. the key is for any layout is the Proportions or Ratio.

My opinion. 

When setting up your document, you should include the following measurements and settings:

• Most standard brochure measurements should be set to 8.5” x 11″, which translates to 2250 x 3300 in pixels.
• Your resolution setting should also be 300 PPI.
• The color mode should be set to CMYK (not RGB).

8x   size   resolution