Effects of the particle size of a suspended zeolite catalyst based on the commercial MFI-type (high modulus zeolite, HMZ) zeolite on its physicochemical characteristics and catalytic properties in the conversion of dimethyl ether (DME) in a three-phase system with a slurry reactor are studied. In order to gain insight into the relationships between the physicochemical characteristics of MFI-type zeolites and their catalytic properties, the textural, acidic, and catalytic properties of HMZ zeolites and synthesized nanocrystallites of the zeolite with the MFI structure are compared. It was found that, in the conversion of DME in the slurry reactor, the catalytic properties of the MFI zeolite are strongly affected by the size of particles of the suspended catalyst: on passage to nanosized suspensions, the activity of the catalyst grows by several times. Reduction in the size of catalyst particles leads to an increase in the yield of hydrocarbons and a decrease in the yield of lower olefins. At the same dispersity of suspensions, the catalytic properties of suspended catalysts are considerably affected by the distribution of acid sites over strength in the studied zeolite sample and the fraction of amphoteric sites in them.