[Manolo Martinez]

I added a 25*40 garage and will be putting AC in most of it, wash room, welding box, game room, and freezer plus off coarse all the regular outlet and lights about 4 circuits. I think I need a 125amp subpanel and have room in the main panel for a big breaker to service the subpanel.
Can I use it this way and what size wire do I use to run about 55' from the existing panel? It will go in the attic.

[donf]

Can we start at the main panelboard first?

What is the incoming amperage to your main panelboard? Next, has the manufacturer of the main panelboard made allowances for a 125 Amp dual pole breaker to be added to this panelboard? If the manufacturer has not said in their instructions or specifications that you can use a 125 amp breaker than you cannot. You will usually find a, " largest breaker " in the specifications.

As to your garage, figure out the amperage needed for lighting, receptacles, heating / air conditioning (pick the higher of the two loads) then determine what amount of amperage you need.