Alright, so your machine isn't that limited.I use 3x3 currently, but would prefer bigger maps if possible... otherwise the distance between fortress and map edge is pretty small.
Standard 4x4 size is usually large enough for the typical fortress, especially with half-intelligent zoning (industrial floor with many staircases up or down to storage floor, residence floor directly above main floor, etc.) favored over messy single-z sprawls.