Why should those two have an anticorrosion effect on metals? Anyway, any size would work...

yes, any shape can be used. but a flat metal surface  can make you examine the corrosion phenomenon more easily. choose low thickness can give you a more precise weight loss result. 

If you want to measure the corrosion current electrochemically, I would suggest to use 10 mm dia. round shape samples. when you mount them using epoxies, since you have no edges, you will avoid crevice corrosion and also it is easy to polish them. 

first: very important is the environment  use for corrosion and inhibitor efficiency testing,

second: also very important is the method use for investigation of corrosion and inhibitor efficiency.

so the metals can be steel, low allow carbon steel, Cu, Al,.... different allows, in respect of the corrosion field under investigation and the shape and the size of the samples is closely connected with the method used for investigation.

Usually, for electrochemical investigation the specimen are with cylindrical shape (with diameter on the average from 5mm to 7mm ), fixed inside the teflon tube with epoxy resin. 

For weight lost method, usually used parallelepiped-shaped samples, in terms of size, can be different.

Shape of steel is up to you. But size of used plate is depend on volume of corrosive medium during exposition. ASTM standard define ratio of surface of sample to capacity of corrosive medium.

In this corrosion study, what concentration(1M, 2M,...so on)  of acid( you are using in your study. I am using 1M HCl in my corrosion study in weight loss method. I used the coupon of low carbon steel in squar shape. I used the coupons of 1" length, 1" width and 0.5 mm thickness. The study I performed in 8 hours  for weight loss study. 

For electrochemical study we used the coupon size 1 cm length, 1 cm width and 0.5 mm thickness. The study was completed in 5 minutes in HCl medium.

Hi Tosin,

 Coupon Size depends on the technique which you are using to study  and characterize the inhibitor efficiency ,whether electro chemical polarization (impedance spectroscopy) or weight loss immersion testing. You may refer to "NACE standard TM0169-2000, Laboratory corrosion testing of metals" for coupon size and shape details.

For this purpose you can use the ASTM standards like ASTM G170 - 06(2012).