How big is an oil lamp?

It's somewhat a vague question this, but there's a handful of answers. The best could be based off the generic oil lamp used in the late 1800's until gaslight became popular.

In that type of lamp, they were first usually made out of brass or bronze, not the glass people commonly see with kerosene lamps. This was partly due to safety, but mainly due to the fact that it was cheaper to construct them that way.

The lamps would have a rather curious shape to them, with the widest point being at the top, tapering down into a narrow "waist" and then flaring some at the base. Since oil was hard to come by, the larger the lamp the longer it could burn, and thus the more efficient it was.

Your average lamp of this period, the type used in a living room or parlor, would be about eight to twelve inches wide at its widest point below where the wick is located, and tapering down to four or five inches at its waist, before flaring to six or so at the base. With the shade and chimneys on them, the lamps would stand about sixteen to eighteen inches tall; though larger lamps did exist.