Sydney's small rooms are the reason this format does so well here. A restaurant in Surry Hills with no stage, a Woolloomooloo boat with a weight limit, a harbourside terrace whose licence caps amplified music at a level a drum kit would blow through in one bar: none of those can take a band, and all of them can take two players in a corner.
What you are choosing between is pairings, not sizes. A voice with a piano puts songs people recognise into the room; a guitar with a saxophone stays further back and lets the conversation run. Listen to how the two respond to each other on the clips, because with nobody else on stage there is nothing covering a pair who do not know each other's playing.
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Around Sydney the usual pairings are piano or guitar with a singer, guitar with a saxophone, or a chord instrument alongside a double bass. They do noticeably different jobs: put a voice in and guests start recognising songs, leave it out and the music sits behind the talking. Shortlist on the combination rather than the group name.
For a seated dinner or drinks up to roughly eighty people it is fine, usually with one small speaker lifting the vocal. Outdoors on a lawn or a rooftop the sound goes straight up and away, and two players start straining to reach the back row. Give them the headcount and the setting, and if they tell you to add a third, believe them.
Two players and the number of hours set the base, then travel across the city and whether a small PA comes with them. Since jazz is priced by the chair, this is the least expensive way into live jazz and a couple of hours often lands below what people brace for. The from-prices on this page are what Sydney pairings are quoting at the moment, so start there and send your date for a firm figure.
It is one of the few live music options that genuinely suits a harbour cruise, because two people and a compact setup fit where a band and its gear cannot. Check the power situation with the operator, since some vessels want everything running off a single outlet, and warn the players about the boarding time. Gear going up a gangway in a swell needs longer than a footpath does.
A lot of Sydney venues carry a cap on amplified music, and inner-city terraces have neighbours a few metres away. Two acoustic players can usually work inside a limit that rules out everything else, so hand over the restriction early rather than hoping nobody measures it.
A keyboard, an amp and a double bass still have to get from a car to the room, and a CBD or eastern suburbs address with no loading zone and paid parking makes that a real part of the job. Say where the closest unloading spot is when you enquire.
If one of a five-piece is unwell the night still happens. Lose one of a duo and half your music is gone, so ask what their arrangement is for a late replacement and expect a professional to answer without hesitating.