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Please check the building code at your area/country, or you can check the ASCE (Min. Design Loads for Buildings and Other Structures), and UBC code and standards publications.

ASCE = American Society of Civil Engineers & UBC = Uniform Building Code

Note. At ASCE and UBC, I'm not sure that you will find something helpful which replying your question. Since I'm a mechanical design manager, and I used to use the both codes in calculation of forces due to wind and earthquake affecting on petroleum and petrochemical equipment which are considered a structures.

To answer your question, large enough for the result to be meaningful. I could test a .001" dia sample of steel, but the properties would not mean anything if my actual part were 1". Too large, and the test becomes difficult. Most homogeneous products use samples between 1/4" (6 mm) and 1" (25 mm). The length should be short enough that failure is by yield rather than buckling.

Don't try this yourself unless you have the proper equipment. Factors like eccentricity loading, non-flat surfaces, and load application rate can effect the results. Send the material to a test lab. It won't cost much, and you can be sure the results are accurate.

Additionally, if this product is to be marketed, third party (non-biased) testing is essential. Sure, do your own in-house testing during R&D, but when you have your final design, have an independent lab test to some applicable standard.