I've read that using large implants could result in a wider scar. What size implants will worsen breast lift scars?

Thank you for the thoughtful question.    There is no uniformly accepted "large" breast implant. As you can imagine, what may be too large for one patient (for example small framed) may be just right for a wider framed patient.   Also, there are many variables, besides the size of breast implant utilized, that will determine exactly how a patient scars with breast lifting surgery.Generally speaking, patients undergoing the breast augmentation/lifting combination operation should understand that it is significantly more complex than breast augmentation surgery only and that it is associated with a significantly higher risk of complications and need for additional surgery. 
  As you have noted, patients undergoing the combination breast augmentation/lifting surgery should understand that there are limits as to how large of an outcome can be achieved, given that breast lifting surgery does involve breast skin removal. On the one hand, breast augmentation surgery is expanding the breast skin “envelope” while breast lifting is (by definition) tightening up the breast skin envelope. These 2 forces are counteracting each other. 
Therefore, it becomes important to remove the appropriate amount of breast skin and to use the appropriate size/profile of breast implants to balance these 2 forces appropriately and to allow for achievement of the patient's goals while minimizing risks of complications. 
In my practice, "tailor tacking" is an important part of the mastopexy and mastopexy/augmentation procedure. It involves TEMPORARY  closure of the skin (for example with temporary skin staples)  to double check the amount of skin that is to be excised during the procedure. This maneuver is done to ensure that enough skin, but not too much, is removed. On the one hand, it is important that enough skin be removed to achieve the aesthetic breast lifting results the patient is looking for; on the other hand, is important not to move too much skin creating tension upon closure and potential wide scars and/or implant exposure. .
 
Also, generally speaking, the best online advice I can give to ladies who are considering breast augmentation/lifting surgery (regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work. 
2. Again, have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining which operation and/or breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "C or D cup" etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate. The use of computer imaging may be very helpful during this communication phase.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery. The use of the tailor tacking technique and the use of temporary intraoperative sizers with the patient in the upright position makes selection of the best breast implant size/profile relatively easy.
Best wishes.

An implant that is too large for the skin volume will cause stretch and pressure, and thus result in a scar that tends to widen, or in worst case scenario, open up.  So, there's no set answer, which is why you won't see it anywhere, but 400 is probably ok.  Now, if I or the PS were to make the lift/reduction too small and shrink the skin envelope, and then try to stuff a large implant in (relative to that size), then yes, it could cause widening.  There is a very fine balance.