Dimension is an option of several functions and denotes the number of space-time dimensions. Possible settings are: 4, n, d, D, … , the variable does not matter, but it should have head Symbol.
Options[ScalarProduct]\{\text{Dimension}\to 4,\text{FeynCalcInternal}\to \;\text{True},\text{SetDimensions}\to \{4,D\}\}
ex = ScalarProduct[m, n, Dimension -> d]m\cdot n
ex // StandardForm
(*Pair[Momentum[m, d], Momentum[n, d]]*)