As an adult, I don't think I've stayed the same size for more than a year or two at a time. My weight has gone up and down (more often up) so frequently, that I never know from one season to the next what items in my closet will fit. Since my weight has fluctuated so fast and by large amounts, I haven't typically purchased clothing with the thought that it would be a staple in my wardrobe or something that I could wear for many years to come. I haven't bothered with many of the "must have" pieces because I never could justify the expense of an investment item such as a basic navy blazer or a really nice cashmere sweater. Yes, I've had some classic pieces such as black slacks, a white button down, etc. that can be purchased at reasonable prices and I would just purchase a different size as needed.
I look forward to wearing the same size from one season to the next. It may seem like a silly thing, but I'm sure others of you understand this. It will be fun to build a wardrobe with a sense of permanence rather than basing my purchase on what will work for this season only.