How to get current from radio signal circuit
When you know the load (resistance) and the voltage across it, you can calculate the current with V=IR.

Assuming your multimeter has a 1 Megaohm input, that means you are getting 1.4 microamps (uA) -- This agrees with @PeterBennett estimate of microwatts. Power = V^2 / R = 1.4*1.4/1E6 = 1.96uW. Seems pretty good for a first try!

Depending on how precise your meter is, you could try a 1K load, we can estimate this would see 0.044V across it.

Edit: In addition to trying different known loads, see this post for tips on determining the input resistance of your meter -- If it is a decent new one it might be higher than 1Mohm, in which case the power you're getting could be less than a mirowatt.