I have a problem regarding the power rating of my metal film resistors. How can I determine which power they can withstand. I tried looking at the diameter but the 1/4W, 1/2W and 1W resistor all have the same size.
Usually power is only indicated for >5W.

I build a simple 4-point resistance meter with 5V, 12V, 24V input voltage. Some 220Ohm resistor work fine, but others get extremely hot. My assumption is that some are rated 1W and some the standard 1/4W, but they have the same color coding.

I have a problem regarding the power rating of my metal film resistors. How can I determine which power they can withstand. I tried looking at the diameter but the 1/4W, 1/2W and 1W resistor all have the same size.
Usually power is only indicated for >5W.

Through-hole resistors? The power rating for these assumes soldered to a PCB with short leads, note.

Most are 0.4W I think, and fit a 0.4" pitch. 1/2W can be larger and don't fit 0.4" easily. 1W are much bigger
and easily distinguished. If you try to use a common 0.4W resistor at 1W it will discolour and probably smoke. Power should always be given for any resistor from the lowliest 0603 surface mount part to something that
needs a trolley to move around.

I build a simple 4-point resistance meter with 5V, 12V, 24V input voltage. Some 220Ohm resistor work fine, but others get extremely hot. My assumption is that some are rated 1W and some the standard 1/4W, but they have the same color coding.

All 220 ohm resistors will generate the same amount of heat at the same voltage, different sized resistors will see different temperature rises though.

I measured the size with a ruler only. Using my caliper I have different diameter values and using

I can determine the power ratings better.

Therefore my resistors have 1/4W and the bigger ones 1W. I have to measure the smaller ones with the lower voltage, where the bigger ones can withstand at least up to 12V.

I noticed you mentioned that your resistor can withstand 12V and would like to explain something. Resistors are rated for power dissipation not voltage.

Power is measured in watts and it is the product of current and voltage. Your 1 watt resistor can take about 83mA at 12V. At 100mA or higher you would be risking failure. 1(watt)/12(V) = 0.0833 A (about 83 mA).

To calculate wattage you will have to use Ohm's law to determine your current based on your voltage and resistors as you are using.

Please get familiar with Ohm's Law as this is one of the basic foundations of working with electronics.

Qdeathstar:
Resistors are also rated for maximum voltage.

good point Qdeathstar I usually ignore this with low voltage application in AVR projects... Everything has a limit. The correct observation in this case, is that at 12-24V what is critical is power dissipation not the voltage.