Size-controlled synthesis of two-dimensional (2D) catalysts with low stacking numbers and small nanoflake lengths is crucial for promoting the catalytic performance in diverse heterogeneous catalysis. Herein, we report a facile and general "surface curvature-confined synthesis" strategy to modulate the slab lengths and stacking numbers of 2D transition metal sulfides by controlling the strain induced by different surface curvature of supports. An efficient NiMo sulfide with shorter slab length (average 3.71 nm), less stacking number (1-2 layers) and more edge active sites is synthesized onto ZSM-5 zeolites with the average size of 100 nm, which shows superior k_HDS value of dibenzothiophene (14.05 × 10^-7 mol/(g·s)), enhanced stability up to 80 h, and high direct desulfurization selectivity (> 95%). This design concept is also proved to be generally applicable to modulate the slab lengths and stacking numbers of other 2D catalysts such as MoS₂ and WS₂ nanoflakes, which shows great potentials for developing more ultrasmall 2D catalysts with controlled sizes and excellent catalytic activities.