Use "as high as could be" not "as high as can be". As you stated, this is something that does not exist. You don't know that it "can" be that high. But you wish that it "could" be. So you use "could" for the hypothetical (not real) thing.

Use "as high as could be" not "high as much it {could/can} be".

As you stated, this is something that does not exist. You don't know that it "can" be that high. But you wish that it "could" be. So you use "could" for the hypothetical (not real) thing.