Please can someone help me out about the problem of determining variable capacitor values? I'm trying to make a radio circuit and i don't ever get a clue as to what value of Inductance i should use because there isn't any value marked on the variable capacitors. Thanks.?

 

Well, you could use a known inductance in parallel with the unknown capacitance to create a resonant tank circuit. Use that in the feedback path of a high-speed opamp and measure the output frequency.