This has always driven me nuts. It applies to cases where a Yarnover is preceded by a knit stitch and followed by a purl stitch, and then that string is mirrored elsewhere in the row (the Yarnover preceded by a purl stitch and followed by a knit stitch). I looked at tutorials, watched youtube videos and none of them seemed to equalize the size of the eyelets. That was until I began working on Whippoorwill, a shawl that has mirrored YOs throughout, and they MUST match in size or the shawl will come out with a longer wing on one side than the other. In the stitch legend, the designer describes a method I hadn't seen before.
Important Note: Bear in mind, on this particular shawl, YOs are worked on every row (both RS and WS) serving as wingspan length increasers. On the RS rows, the pattern states that "no modifications are needed." It's on the WS rows that this treatment is specified; however I think this also may apply universally if the key to equalizing eyelet size is not twisting the yarn on one of them on the next row. So if working in stockinette, it would just mean that since your "next row" would be a purl row, where on the below shawl-specific example it says "knit into the back of the YOB to open up the eyelet," in general you would instead purl that stitch on the next row in whatever way necessary so that your yarn doesn't get twisted in the process (thereby undesirably closing up that eyelet). If working in garter instead of stockinette, I think it would be just as written.
That stated, here's how it goes on this shawl.
For k, yo, p, you bring the yarn forward between the needles over the top of the right needle, and back under the right needle to the front of the work, ready to purl the next stitch.
For p, yo, k, she calls this a "YOB" (yarnover backwards). Here, you bring the yarn between the needles to the back of the work, over the top of the right needle, and back under the right needle again to the back of the work, ready to knit the next stitch. However, on this one there's a critical next step. In the following round, you knit into the BACK of the YOB to open the eyelet. This prevents that YOB from being smaller than the YO it mirrors.
As for Whippoorwill, I've been wanting a really cozy, BIG shawl, purely for warmth (non-lace). So though this was designed for fingering, I decided to make it in the large size, and use Berroco Ultra Alpaca, a worsted weight yarn. My gauge is substantially different on a size 9 needle than her fingering weight yarn on a size 6 needle, and I was counting on that to yield a pretty good size shawl. However, a Big ETA: I need to figure some things out before recommending that size modification because there is a design issue that works well for a small shawl but presents a problem with the size I'm making it. I'm considering a workaround to make this work out in the end, and will blog and/or include Ravelry project notes on it if it does. Or doesn't. It's not the shawl's fault. I'm just bending it into a size beyond what it was specifically designed to be.