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´¡²ú²õ³Ù°ù²¹³¦³Ù:ÌýPlatelets are small cell fragments that are essential to maintaining hemostasis. While traveling in the blood, they adopt a discoid shape that is sustained by a ring of microtubules. To study how this ring sets the size of the platelet, we measured the characteristics of various rings as a function of their size. In resting human and mouse platelets, we find a quadratic law between the size of platelets and the amount of microtubule polymer they contain. We further estimate the length and number of microtubules using Electron and Super-resolution microscopy. In platelets activated with ADP, the marginal band bends as a consequence of cortical contraction driven by actin. Larger platelets have a higher propensity to buckle. Moreover, this elastic buckling response is reversed by elongation of the marginal band. Lastly, we propose a mechanism by which the marginal band can produce outward forces and derive its scaling behavior using simulations.