Hi there and welcome to Electronics Point
You've made a good start by measuring the signal levels from the phone.
The assignment hasn't told you the basis for working out the gain, but I would say one reasonable way would be to ensure that the OPA551 won't be damaged (by having too much current drawn from it) even when your phone is set for maximum volume with a continuous tone at 100% amplitude.
So you need to work backwards from the output. Assume the speaker is roughly resistive and has the specified resistance (unless you have been taught more detail about real world speakers). You need to make sure the speaker won't try to draw more than the rated current from the OPA551, right? So what voltage does that current correspond to? Make sure you use the right units (volts RMS, volts peak, or volts peak to peak).
Then, what gain will the OPA551 be operating at? And what is the maximum input voltage you want?
You know the maximum voltage available from the phone, so now you can calculate the gain of the filter stage. Do you know what configuration you will use for the filter? Does the filter stage have a limit to the minimum passband gain? If it does, is there a simple way to adjust the input signal to suit it?