When I started to address the acoustic issues of the garage, I thought I should borrow some carpets from another room and cover the walls with egg cartons.
What I learned since is that carpets and egg cartons will absorb only the higher frequencies, which have very short sound waves, and not affect the lower frequencies. This is a diagram that shows the effect of egg cartons. The lower frequencies (notes) will sound (and reverberate and be reinforced) as before, but there will be a pronounced dip in certain of the higher frequencies. What one wants is equal absorption, and for that purpose size (or more accurately the depth and density of the absorbing material) matters.
It therefore turned out that the simplistic carpet-and-egg-carton treatment I envisioned would not be adequate. Selling my old car, painful for me, did free up the garage space and allowed a small budget that I would not have otherwise had. I bought a nice guitar and some gear but did not expect acoustic treatment to be so complicated. I was lucky though in that one of my old-car friends was in the business of sound treatment and would help for free.