Sub Size Specimen

Your question is FAR to broad and unspecific to answer here.

The material tested will influence the results.

Define "sub-sized" specimen.

Are you testing in accordance with a specific specification?

If so, the parameters will be defined or referenced there.

Use of Miniature Tensile Specimen for Measurement of ...

Small Tension Test Specimens - ASTM (testing materials) Code ...

Otherwise the topics of stress values, yield stresses and ultimate tensile strength are well known and well defined in literature.

The question makes no sense if you are using ASTM standards.

The specifications specify the manufacture of the specimen, method, environmental conditions, etc.

If the OEM specifies something different than acceptable test methodology, then you have to follow what he wants. Testing is performed to limit liability and verify the grade, and class, of the specimen. Testing is, usually, very costly, and time consuming. Testing by another method may invalidate this liability limitation and still be very costly.

If you are the OEM, why are seeking to vary what is considered to be acceptable?

Because of the smaller cross-sectional areas of sub-size specimens, force and energy absorbed will be less than normal. However, stress (force per area) will not change, at least not by much. The Code to which you are working should state what adjustments are needed in any calculations.

When one performs mechanical testing (such as tensile testing), the gauge length of the test specimen is usually specified. The reasons for this are directly related to your question, and yes, gauge length does make an important difference.