There is precedence for cells being that large. I can't remember the organism name, but there is actually some macroscopic single cell organism. It cheats by having multiple nuclei, among other things.
The article explains that the huge size for a bacteria seems to be enabled by 2 features, first a large water-filled vesicle inside, so that the living part of the cell is around the periphery of the cell, within short distances from the cell membrane, and second, a special compartment for DNA which provides some of the functions of a nucleus in an eukaryote (nucleate) cell.