Resistor Magnitudes in Voltage Dividers
Theoretically, any correct ratio works but of importance is not loading the 5V source too much that it sags a little i.e. don't choose resistor values that are so low that what you measure changes due to the introduction of the two resistors. 

Next is consideration of what is measuring the output from the potential divider - you don't want the resistors to be so high in value that the impedance of the instrument or circuit changes the ratio. For instance an oscilloscope might have an input resistance of 1Mohm - using a 100k top resistor and a 900k bottom resistor will not be good. Try to aim for 0.1% (just a figure that is reasonable to me) so that the bottom resistor would be about 900 ohms and the top resistor 100 ohms.

Some DVMs are 10Mohm input resistance so a divider formed from 1k and 9k would be OK.