Materials: In order to avoid the presence of Bragg peaks in the spectra, we use cans made of Vanadium (which is a incoherent scatterer) or of a TiZr alloy (which forms a null matrix).
Thickness: Usually we run aqueous liquids in 1 mm thickness can and powders in 2 mm thickness cans. This allows an optimal single to multiple scattering ratio.
Dimensions: Our beam is about 30 mm diameter, therefore our cans have usually at least 35 mm diameter or sides. This typically means you need about 1.2 ml of sample.
More details are available through your local contact, who can help you in deciding which can is best used for your experiment.