The fixed dimensions of the most common "small square" bale are 14" by 18".
The variable dimension is settable at 12" to 52" on many small square balers.
A bale length of 36" is fairly common because 18" is half that and bales can be interleaved when stacked to make the stack more stable.
Some more expensive small square balers produce bales with fixed dimensions of 16" by 18" and usually a variable dimension of 12" to 52". These larger small square bales are less common.
A 14"x18"x36" bale typically weights 60 to 70 lbs. Weight depends upon forage type, baler and moisture content. Ideal moisture content for baling small square bales is 15% to 18%. Baling small square bales when hay has a moisture content above 20% can result in more fungus growth and a potential for spontaneous combustion fires. When hay is baled moist two bales should be tipped and leaned against each other to form an A-frame so that their surface is exposed to the air to dry. This can be done in the field on dry days or under cover if the reason you are baling moist is weather.
Smaller bales hold together better, dry quicker when left with air space around them and are easier for youth and women to lift. A bale size of 14"x18"x22" will weigh about 37 to 43 lbs. This is a safer bale weight for many people to handle - less chance of an injured back. A bale of this size is about 2 days of feed for cattle at 20 lbs per day. Horses typically eat about 22 lbs of hay a day so a bale size of 14"x18"x24" might work better with horses. A bale size of 14"x18"x36" is about 3 days worth of horse feed. Macho bales - 48" to 52" - tend to come loose and are definitely difficult for some folks to handle though hefting them gracefully may impress a lady.