Someone I know wants to start a small business. He’s eager. He’s enthusiastic. He’s the right age. (While any age can be the right age, research reveals that older entrepreneurs tend to be more successful, even with tech startups.)  He has an idea, and he’s thinking big. “Disruptive big,” as he puts it. Problem is, his idea involves underwear. Not that an underwear startup is a problem in itself; Mack Weldon launched with underwear and socks. But he hasn’t come up with new fabrics. Or new styles. Or new designs. Or a new marketing twist. For him, it’s all about market size: Everyone wears underwear. Yet, paradoxically, that’s a real problem. As Mark Cuban says, “When…