Hi,
The procedure is as follows:
Let's say you want to build the circuit like here:
**broken link removed**
1. You use minimum length of transistors (are the fastest and takes smallest amount of place).
2. Use any width of transistors as you like. Start for example with 10 um.
3. Take any current you want. For example, 50 uA for current source of differential pair.
4. Simulate the circuit and write down its parameters (power, bandwidth etc.).
5. Compare the parameters with the required spec and adjust your circuit by changing currents values, widths, lengths etc.
Generally, you like to start with small current values, shortest lengths and small widths. Why use a lot of current and take up a lot of space (large L and W)? And then you start tuning if you do not meet specification.
Hope it's helps.
BTW. I think every designer has his own routines, so treat the above just as an example.