Resistors

The targeted resistance value of the component.

The variation in the resistance value by manufacturing processes. 1% resistors are very common and we used these whenever possible. 0.1% is used for high precision measurement. 5% resistors are typically used when a higher power rating is required, as higher power rating and high precision can be expensive.

The power rating of a resistor is dependant on the maximum allowed temperature of the resistor (typically around 70 degrees in the datasheets). Make sure to have at least a small safety factor for the power rating. P = I²R

The physical size of the resistor. 0603 resistors are used whenever possible, as they are easy enough to hand solder and a good compromise for space saving.

SMD devices are standardised by the EIA, and the numbering represents the length and width in hundredths of an inch(so 0603 is 0.06 inches long and 0.03 inches wide etc)