Congratulations on keeping the rhubarb going in the garage. It must be in good soil in a good place. Thin stalks are typical of a first-year plant. As noted above, it is best not to harvest in the first year. You can repot it; this should not harm the plant. Set it in plenty of organic/composted matter and it should thrive. As for how much would you need for a pie, this one requires 2 1/2 cups: https://www.almanac.com/recipe/strawberry-rhubarb-pie-coconut-crumb-topping
You can see more rhubarb recipes, using varying amounts, that might appeal to you, too: https://www.almanac.com/content/best-rhubarb-recipes
Congratulations on keeping the rhubarb going in the garage. It must be in good soil in a good place. Thin stalks are typical of a first-year plant. As noted above, it is best not to harvest in the first year. You can repot it; this should not harm the plant. Set it in plenty of organic/composted matter and it should thrive. As for how much would you need for a pie, this one requires 2 1/2 cups: https://www.almanac.com/recipe/strawberry-rhubarb-pie-coconut-crumb-topping
You can see more rhubarb recipes, using varying amounts, that might appeal to you, too: https://www.almanac.com/content/best-rhubarb-recipes