The smallest viable market you can dominate first, before expanding out. A beachhead works best when it is an activity or identity your customer already has, not a product category you have invented. Owning a tight niche gives you concentrated word of mouth, sharp messaging and a defensible base: Quad Lock owned cyclists first, then expanded to runners, motorcyclists and drivers. The founder mistake is going straight for 'all active people' - a huge TAM means nothing if you cannot win a definable slice of it first. The useful question is never how big the market is, but how small a slice you can dominate.
Benchmark. As a rough guide, a beachhead should support $5-10M in revenue on its own; if it needs $50M+ to be viable, it is probably too broad to start. Plan on capturing 0.1-0.5% of it in year one - Quad Lock's 2-5% was an outlier.