You need a 31-step transmission tablet in your typical negative format to enlarge it (mine is 4x5, which is also useful for other tests where contact printing is the way to go). If you don't have a densitometer, you also need a 21-step reflection tablet to measure your results.
Could you comment on using a 4x5 transmission tablet as an in-between if your typical negatives are 6x6 medium format or 8"x10"?

Tom.