Downsizing to smaller rig, battery capacity

Looking at your old setup, a pair of GC=2 and solar, how did that do?
If you were not drawing that down to less than 60% of a full charge, then cutting it in half is about your minimum.

Two GC-2 batteries with 210-230 amps will allow for the needed 64 amp hrs for the O2 concentrator and the 32 amphrs for the CPAP but you will need to top off daily, not getting back over 90% with that will leave you lacking after only a day or two.

Are those amps at 12V-DC?

I'm guess yes. If they are 120v-AC, not a chance.

Assuming 8hr use between charges...that's around 96amp-hr...assuming nothing else is drawing a couple golf cart batteries should do the trick but that assumes you have a means to completely recharge the next day.

Have no idea what you mean by "standard size batteries". Deep cycle 12 volt batteries are Group 24, Group 27, Group 31, sometimes also a Group 29. The greater the Group number the larger the battery is but also the greater is it's capacity. Since a CPAP draws quite a bit of power, especially if you need it's humidifier running also, I'd go with the larger Group 31 if there's sufficient space to mount them. Otherwise you might want to switch to a pair of 6 volt GC-2 batteries which in series will offer about the same capacity as a pair of Group 31 12 volt batteries wired in parallel, and although taller will have a much smaller footprint, about the same as a Group 24 12 volt battery.