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.

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.