Girth over length, every time. Length is the number people fixate on and the one that matters least. A body accommodates length by angle and position. Girth is the measurement you actually feel, and it is the one that decides whether something is comfortable. Measure a toy you already like and shop to that number.
Firmness is the hidden variable. Two silicone toys of identical dimensions feel completely different if one is soft and one is dense. Softer forgives more and is easier for beginners. Firmer transmits pressure and is what people want for G-spot and prostate work. This is rarely in the product photo, so read the description.
Decide about the base before you buy. A suction cup mounts to a wall or floor for hands-free use. A flared base is required for anything anal and for harness use. A plain handle is neither. These are not interchangeable and you cannot add one later.
Curved for internal, straight for everything else. A pronounced upward curve is built to press the front wall. If you are not chasing that specifically, a curve can just get in the way.
Match the lube to the material, not to habit. Water-based works with all three materials here. Silicone lube lasts longer but degrades silicone toys, so save it for glass and steel. Getting this wrong ruins the toy permanently.
Non-porous is not optional. Silicone, glass and steel can be sterilized and will last for years. Jelly, PVC and anything vague about its material cannot, and it will not. We do not carry those, and it is the one thing worth checking anywhere else you shop.
