Find the largest circle in the closed curve, center your text to the center of that circle, and scale it’s up using the diameter of the circle → such that the circle that passes through the points of the bounding box of the text is equivalent to that largest circle in the closed curve

The case you showed is a completely different one than the T-shirt from your original post.

In any case depending on the scenario, you will oscillate between finding the largest circle or the largest rectangle or even the smallest/largest linear distance between some geometry you’re working with.

In the case of the two parallel lines from your reply I would scale the bounding box of the text until it reaches both lines.

But for the t-shirt case I would use the largest circle inside the T-shirt. Finding it is rather trivial. The largest circle will always be on one of the vertices of the medial axis. So use a pluggin to find the medial axis, extract the vertices, and query which gives you the largest circle