I have a 125 Amp main panel with a 70 Amp dual pole breaker in it. It feeds a VERY old sub panel which has about a dozen old circuits on it. I haven't had any problems with the sub panel (yet). Unfortunately, the 125 Amp main panel is full (12 single pole spaces... mostly with dual pole breakers - AC, Surge Protector, Dryer and a few tandem single pole 20 Amp breakers). I priced a new 200 Amp panel at about $2000 which I can't afford right now.
Can I just upgrade the sub panel to a new one (which is probably needed anyway) giving me an extra space or two for future growth? I won't go crazy adding new circuits, but just in case.
Also, if I leave the 70 Amp breaker in Main panel, what size box is maximum I can get on that? I don't see a lot of 70 Amp sub panels out there (if any). Maybe a 60 if enough spaces? I won't obviously max out the sub panel. Can I not go to 100Amp box and just not fill it? I will get an electrician to do the work.. But for right now, I just want an idea of what is possible because when I had 4 electricians come out to look at upgrading service, they all had different ideas and different quotes. Thanks!

You can have a 42 circuit panel with a 70 amp main breaker, if you like. The panel can be full of circuits, you will always be limited to a total maximum of 70 amps due the main breaker.

You must keep a 70 Amp Main, due to the wiring feeding that breaker is rated only 70 Amps.

Any decent electrician that understands services will understand and should help you.

If you start with a 200 Amp 42 circuit panel, later when your ready, a new 200 Amp service entrance can be installed and connected to the new panel already in place.