A long sleeve tee is not just a short sleeve tee with longer arms. Sleeve length, armhole depth and cuff finish all change how the garment fits and how it looks when layered. Many brands start from their existing short sleeve block and discover that the sleeve twists, the cuff stretches out or the shoulder feels tight when the arm is raised.
Give measurements for sleeve length from the shoulder point or the center back, the cuff opening, and the bicep. If you want the sleeve to stack at the wrist, say so, because that requires extra length beyond a standard fit.