Other things being equal, a longer tonearm is always better, with one exception: linear tracking arms.

The reason you want a long conventional arm, or a linear tracker of any length, is to eliminate tracking error and therefore tracking distortion.

When a tonearm moves across a record, it does so in an arc, because the arm is fixed at a point that becomes the radius for arm motion across the record. Ideally, the tonearm would move in a straight line across the record. That isn’t possible, except in the case of the linear tracking arm, seen on decks like the Technics SL-10 for example.

For conventional arms, the stylus will scribe an arc across the record, the curvature of which is determined by the length of the arm. The longer the arm, the greater the radius, the gentler the arc and the lower the tracking error, and therefore distortion.

This is the single reason why many of the best tonearms are 12 inches long in the old money. Very good arms are usually 10 inches long or close to it and the standard length is around 9 inches. There are some excellent 9-inch arms, but this is about as short as you want to go before tracking error causes significant tracking distortion.

You’ll occasionally find turntables with short tonearms, mostly sold in department stores. I’ve worked on a few Gold Notes for example with 8-inch arms. Short arms allow for small turntables, but the compromises are too great and I strongly suggest you avoid anything with a less than 9-inch tonearm.