@bunnyknight
Yes, there are great images of Europa and the like but that is not from a telescope based on Earth or in space, that is a probe flashing by a few hundred Km above those moons and such.
If you take as base the venerable binoculars around for centuries, the 7X50, 50 mm lens at 7 power puts the image to match the pupal of our eyes, so a 500 mm lens would give 70 power and a 5 METER lens would give you 700 power, a 50 meter lens, 7000 power.
Of course you can get more but the image doesn't fill you whole pupal at that point, so you can get 70,000 power out of that alleged 50 meter scope and such but to fill your pupal at 1 million power, a 500 meter scope would etch out 700,000 power, so maybe a 700 meter scope could fetch 1 mega power but that is a tad bit ahead of our ability to make right now.
On the other hand, the resolution of multiscopes linked together can make effective mag as if it was a single mirror the size of the distance between scopes but so far we cannot get scopes to be in phase at 700 meter distance apart, but even that may be changing with some real advances in telescope tech, using quantum physics that may allow scopes to combine at kilometer separation but that is probably 20 years away from real scopes. The one thing about separated scopes, the effective magnification goes up but not the ability to capture light that a real 700 meter scope would give, the light gathering power is just how much glass reflects the light.