Specifications of this Signal Generator
On the second page of the data sheet, there's this table



83650B has a maximum output power (into 50 ohms) of +10dBm below 26.5GHz, +5dBm below 40GHz, and +2.5dBm above 40GHz. Unless you have the attenuator version that is which reduces the power.

So what's +10dBm? 10dB above 1mW, or 10mW. 10mW into a 50 ohm load is an rms voltage of 0.707 volts, or a current of 14.1mA.

If the output is levelled properly, then into an open circuit it will deliver twice this voltage. Into a short circuit, it will deliver twice this current.

For unlevelled operation (ie turned up to the max), you're likely to get an unspecified amount more than these figures, which will vary with frequency.

5dB in linear ratio is 1.778, reduce those voltages and currents by that factor for operation 26.5 to 40GHz.

There is a way to get a little more voltage into loads that have a higher resistance than 50 ohms.You still get the same power. There are two ways of transforming the impedance. The first is to buy an RF transformer, from people like MiniCircuits. The second is to use a quarter wave of intermediate (square root of source*load) impedance transmission line. For instance, if you had a 200 ohm load, a \$(2n+1)\frac{\lambda}{4}\$ length of 100 ohm line would match it at the frequencies for which n was an integer. With four times the load resistance, the voltage into it doubles.