What do you mean by small? and what sort of top bar hive? is it what used to be known as a Kenyan Topbar - sort of coffin shaped? if so, quite simple really -it was designed for African subsistence farmers and due to the fact African bees work differently, did not need weekly inspections - the 'top bars' are just pieces of wood with a channel cut in the centre of each where you can put in a starter strip, bead of wax or leave as it is and the bees will build free comb off that, they raise brood off those as per normal with the usual food arch then they will store surplus food stores on the outer combs (just as in a standard brood box when the colony reduces for winter. The beekeeper then just harvests the combs at the outside of the brood nest and extracts the honey using the crush and strain method