ASTM D638 is the standard test method for tensile properties of plastics, and it includes provisions for testing hydrogels.ASTM D575 is the standard test method for compressive properties of plastics, and it includes provisions for testing hydrogels.

The dimensions of the sample depend on the size of your testing machine and the type of test you want to perform. In general, the sample should be large enough to fit securely in the testing machine and allow for accurate measurement of deformations, but small enough to be representative of the material as a whole. Some guidelines for sample size are provided in ASTM D638, Standard Test Method for Tensile Properties of Plastics, and ASTM D695, Standard Test Method for Compressive Properties of Rigid Plastics.

For compression testing of a hydrogel sample, you can use the protocol outlined in ASTM D695. This standard covers the determination of compressive properties of rigid plastics using a compression testing machine. It includes provisions for testing specimens of various sizes, including Type I specimens with a 50 mm diameter and a thickness between 2.5 mm and 10 mm, and Type II specimens with a thickness between 10 mm and 50 mm.

The test procedure involves preparing the specimen, aligning it in the testing machine, and applying a compressive force at a specified rate until the specimen fractures or reaches a specified deflection. The compressive strength, modulus of elasticity, and strain at yield or at maximum load can be determined from the results of the test.

I hope this helps. Let me know if you have any additional questions.