Nanosized ZSM-5 (< 100 nm) zeolites with high acidity (Si/Al < 15) can bring distinct advantages as catalysts for obtaining aromatics in C1 chemistry. Synthesis of such nanosized zeolites in a cheap and scalable manner remains a challenge. Herein, nanosized ZSM-5 (20-50 nm) with high acidity (Si/Al = 11) was hydrothermally synthesized in high yield using p-phenylenedimethylene-bis(tripropylammonium) dichloride (Pr3N-benzyl-NPr3) as the sole organic structure-directing agent (OSDA). By investigating the solid products formed during zeolite synthesis, it was found that crystallization follows a solid-state transformation mechanism. An in-depth NMR study combined with TGA measurements reveals that, after early electrostatic interaction between condensed aluminosilicate and the ammonium groups of OSDA, ZSM-5 crystallizes around the OSDA. Owing to its high acidity and shortened diffusion paths, such nanosized ZSM-5 demonstrates, compared to bulk ZSM-5, enhanced aromatics formation in methanol dehydration and non-oxidative dehydroaromatization of methane reactions.