"and the sample i mean that the sample should mbe as much smaller for better result."

Why would a smaller sample give better results? Do you have any references?

A small metal cube will have a larger surface area compared to the volume. This means that it will radiate out a larger percentage of heat. This affects your measurements of transmitted heat.

Impurities should most probably affect a smaller sample more because of larger standard deviation in the smaller sample from less averaging over the volume.