Mesoporous carbon (MC) has wide applications, including in drug delivery, catalysis, absorption, energy storage/conversion, etc. Achieving pore size adjustment is an important method for broadening its application. Here, MC with tunable pore size was synthesized by the co-assembly of resorcinol formaldehyde resin, 1-alkyl-3-methylimidazolium bromide and tetrapropyl orthosilicate in a Stöber-like system. The obtained MC exhibits a sheet-like morphology and the mesopore size of MC can be adjusted from 3.9 to 5.7 nm by simply varying the volume ratio of ethanol to water. When the volume ratio of ethanol to water is 0.5, the obtained MC shows the largest mesopores, specific surface area and pore volume. Ascribed to these characteristics, the MC used as a supercapacitor electrode exhibits a high specific capacitance of 258.0 F g⁻¹ at 0.5 A g⁻¹ in 6 M KOH in a three-electrode system, outstanding rate stability and excellent cycling stability, which make it a great potential candidate as a supercapacitor electrode.