Starting Size is 40" wide with or without Fleur Style arms, made in a variety of lengths. To calculate the size you need first consider the distance between dining table legs, then assume each person will need at least 24" or more. In a pinch more people can be crowded in such they do need a full 24", but plan on a minimum of 24" per person.
A bench with arms is difficult to slide into from the side, such as at a table. For dining table use I recommend an arm-less bench.
Seats are available with a deep scooped wood seat (very comfy), or a cushion with your choice of leather or fabric,. Wood seats are easiest to slide into from the end. Wood seats have a continuous scoop from end to end, not a specific scoop for each derriere.