How do you determine implant size with a breast lift? I have wider breasts (14.5) and am concerned implant will be too narrow.

For patients with a wide chest, the moderate plus profile implant works well. Of course, the implant volume depends on your existing breast size and the desired size. When doing a lift, breast tissue is removed from the bottom of your breast to help cinch the lower pole and correct sagging, so the breast is made a little smaller just by doing the lift. (A vertical lift is best. Other methods such as the anchor scar method tend to leave lower pole tissue behind, making the lift less effective). The most common implant shape in my practice is high profile to avoid "side boob." Usually, women with a broad chest select large implants to fit their body frame and provide a good cleavage.

Base width is an important consideration for breast implant selection. We don't want it too wide or too narrow. But there is some wiggle room one way or the other. A breast lift can narrow the breast a little - depending on technique, anatomy - so a narrow implant is reasonable. If you have more breast tissue that is frequently subtracted from the base width measurement.
So it is complicated and a bit subjective.

If the implant is too narrow - it won't fill in the cleavage very well or the upper pole enough. The implant is as tall as it is wide. If it's too wide - it can give too much side boob.

If you're unsure of your implant choice talk to your plastic surgeon - they should be able to adequately explain why they're choosing that implant for you and if it's worth it with your lift.
Or get a few second opinions from board certified plastic surgeons.
Good luck!