Here, we developed magnetically recoverable biocatalysts for enzymatic oxidation of d-glucose to d-gluconic acid with high product yields. The catalyst support is based on nanoparticle clusters (NPCs) composed of magnetite particles and coated with the amino terminated silica layer to facilitate further functionalization. It involves the attachment of the glutaraldehyde linker followed by the covalent attachment of glucose oxidase (GOx) via its amino groups. It was established that the NPCs with a diameter of ∼430 nm attach 33% more GOx molecules than NPCs with a diameter of ∼285 nm, although the surface area of the former is lower than that of the latter. At the same time, the biocatalyst based on the smaller NPCs shows higher relative activity of 94% than that (87%) of the biocatalyst based on the larger NPCs, both at 50 °C and pH 7 (optimal reaction conditions). This surprising result has been explained by a combination of two major factors such as GOx crowding on the support surface which should prevent denaturation (similar to the enzyme behavior in cells) and the enzyme mobility which should be preserved upon immobilization. Apparently, for the biocatalyst based on 285 nm NPCs, the lower GOx crowding is compensated by its higher mobility. The high stability of these GOx based biocatalysts in 10 consecutive reactions as well as facile magnetic recovery combined with excellent catalytic activity in “tolerant” pH range make this biocatalyst design promising for other types of enzymatic catalysts.