It seems to me that the next step up from 11x14 would probably be 16x20, if you are working with a 6x7 negative.
Although the slightly larger 12x16 option is worth considering, if you can source paper.
Here are some things I would consider first:
I'm not aware of any top quality 75mm lenses that are intended to cover 6x7 negatives. It may be that the lens you are using is at the edge of its rendering capacity with 11x14 enlargements, and you would definitely see an improvement if you switched to a top quality 80mm or 90mm lens that is designed for 6x7.
And you definitely need to be sure to align your enlarger correctly.
Are your print drying facilities able to handle the larger print sizes?
Have you examined the framing and presentation issues as well?
Do you use a paper safe? If so, is it large enough?
Do you have a print trimmer that is large enough?
Is your work area large enough to use larger trays?
Do you have a place to store the unexposed paper? Do you have a place to store the unframed prints?
Hope this helps.