Is there a formula I can use to calculate the appropriate RCD size
for a circuit?

An RCD is a device that detects imbalances in the neutral and live flows.
Typically, you use 30mA for a final circuit, or 100mA time delay for whole
house arrangements (only when required). However, I think you really mean
MCB, which is a device that cuts out when a certain level of overload
current or short circuit current has been drawn.

For a resistive load, you divide the 240V power consumption in watts by 240.
The heater specifications should give outputs for 240V and 230V. Assuming
the 1.7kW is rated at 240V, this will give just over 7A. A 10A MCB would be
good here, although 16A MCBs are frequently used. Check the installation
manual to see if it (or its flex) requires protection at a certain level.
The manufacturer's recommendation should override any calculations or
assumptions you make.

The hand drier circuit will also have a reactive element due to the fan
motor. However, this will not be significant compared to the resistive
element of the heater. In any case, the things are typically no greater than
3kW so that they can be fitted on a 13A FCU. Use a radial circuit with at
least 16A capacity, with an FCU connecting between. A device such as this
normally shares a circuit with other devices, perhaps on a 32A ring final
circuit.

If you really do want a dedicated circuit, then it may be possible to use
the 16A MCB and avoid the FCU. However, you MUST check the installation
instructions to ensure that this provides adequete protection to the dryer.
Many will allow this, as continental European final circuits are often rated
at 16A with no subsequent fuse protection, unlike UK systems, and many
manufacturers prefer to have a single version of an appliance to sell
throughout the European market. You may need a DP switch anyway for
maintenance, so little saving will be made.

Christian.