there is one single variable that makes the market big enough or growing fast enough: the goal of the client.
Let’s say for example the goal is to increase revenues by $20M in 2 years, and you are looking to enter a new market. You find that the size of the market is $1B, with no growth for simplicity. Given you would need 2% of the market to reach your goal, from the point of view of the size the market is attractive. However, let’s say that your goal becomes instead reaching $800M in 2 years. The same market is not attractive anymore in terms of size, as you would have to reach 80% market share to meet your goal.
In case a growth rate is present your can follow the exact same approach, calculating the actual dimension of the market by the time you have to reach the goal, and comparing such value with the objective you have to reach.
Hope this helps,