shorcut square bracket to decrease/increase size

Maybe you have a reason for using the English language photoshop version, but you can download the Spanish (Español) language version of photoshop by going to the Creative Cloud Desktop Application.

File>Preferences>Apps>Default install language.

Then in photoshop go to Edit>Preferences>Interface, change the UI Language to Español, then restart photoshop.

I get it now, i think.

Are you using a Spanish language keyboard and/or Windows 10 with the English language photoshop?

The traditional bracket keys of an English language keyboard aren't available to the English language photoshop version if the operating system is Spanish.

The easiest thing would be to download a Spanish set of keyboard shortcuts to use with the

English photoshop version and then the keys for brush size should be ' (reduce brush size) and ¡ (increase brush size)

https://helpx.adobe.com/photoshop/kb/downloadable-plugins-and-content.html

(under Keyboard shortcuts)

On an English USA keyboard that would be the -  (decrease brush size) and = (increase brush size)

Same location as the ' and ¡ on a Spanish language keyboard.

You can use Shift + [] to change hardness. Thats default shortcut. Either way you must use Shift to activate brackets.

So 

[ ] decrease/increase size of brush

Shift + [ ] or { } to decrease /increase brush hardness

In case your not aware you can change the Brush size by

pressing Alt+right-click (Windows) or control+option (macOS), and drag left or right

https://helpx.adobe.com/photoshop/using/using-tools.html#selecting_and_displaying_tools

(under Visually resize or change hardness of painting cursors)

Like the error message in the Keyboard shortcuts dialog states, one can only use single characters (keys) for tool shortcuts.

That is, for example, one can assign G for a tools shortcut. One cannot combine any other keys with the G.