The present work aims to provide a complete engineering solution, an appropriate experimental database, and a brief physical background on designing a miniature specimen geometry suitable for irradiation in materials test reactors such as the High Flux Isotope Reactor and post-irradiation out-of-hot cell testing. The physical limits of specimen miniaturization and a background of the scale factor effect are discussed, and principal limitations are defined. The advantages of modern test methods like digital image correlation, as well as some limitations connected to small specimen size, are analyzed. A sub-sized specimen geometry, “SS-Mini,” is designed; the geometry employs existing irradiation capsules leading to the reduced cost of any irradiation campaign. The new geometry performance is evaluated using a commercial 304 L stainless steel, an aluminum alloy including advanced 3D-printed material, a high nickel 718-alloy, tungsten, and an advanced fuel cladding FeCrAl alloy. Mechanical tests are conducted to compare the engineering mechanical properties (yield and ultimate tensile stress, uniform and total elongation values) and plastic behavior of the proposed miniature specimen with common specimen types for irradiation testing.