I wonder if a similar percentage difference in blade length can have as big an impact on skating.
Not exactly, because the full length of the blade is rarely in contact with the ice (only on deep edges). However, what's similar in skate blades is the primary rocker profile (under the center/back of the blade where your weight sits most of the time). Most blades are either 7' or 8' radius rockers. This is a very small difference as well, but makes a big difference to the feel. 7' rockers make learning turns and stops and things easier as there is less blade in contact with the ice. 8' rockers distribute the weight across a little more blade, and that equates to better speed and stability. Proper technique of shifting your weight to the front or rear of the blade on turns makes turns just as easy, but you have more room for error / wrong technique on 7' rocker blades, which is why they are usually recommended to beginners. In the past figure blades existed with as low as 3' or 4' rockers that favored skaters wanting to be able to make precise marks on the ice in a smaller area, and speed skating blades are flat without any rocker at all. 7' rocker blades are like shorter skis, and 8' rocker blades are like longer skis...somewhat anyways.
Sometimes I feel like the sweet spot on my skates is too far forward, but since this is my first pair of skates I don't have anything to compare too.
Now we're talking about the overall profile of the blade. In addition to the primary rocker, most skates have either one or two lower-radius rockers toward the front. What kind of blades do you have?
This chart gives some visual idea of what I'm talking about:
A sharpener who is not careful will also modify the blade profile over time as well, which is why it's important to seek out a highly-recommended sharpener. As for what works best for you it comes down to skill level and preference and what exactly you're working on. Personally, I've found the best luck with Gold Seal's profile, though presently I have Pattern 99 profile blades. The sweet spot should be farther back on the Pattern 99's, but I personally find it much more difficult to skate on. I suspect this is actually because getting onto the 12" radius rocker is easier on Gold Seals as it comes farther back, whereas on the Pattern 99-profile blades I end up being on the much flatter 27" radius portion when trying to do the things that seem more difficult, since there's barely any room to be on the 12" rocker without dragging the toe pick.
The skate sharpener commented I could fit an 11" blade rather than the 10 3/4" that I have. Would this make any difference in the position of the rocker? I am on Ultima Aspire blades with an 8' radius. Would a 7' shift the sweet spot towards the rear?
See my previous post - I don't think this is a good idea, though again it depends on preference and blade profile.
On spins I tend to rock up on the toe pick and stop the rotation. On back crossovers I also tend to scrape the toe picks on both feet. This may be from bad form and technique but as my experience with skis showed today it could be equipment as well.
Back crossovers I'd blame on technique. Your overall weight should be back far enough that it doesn't matter what blades you use or how long exactly they are (except for the increased "click of death" possibility with overly long blades, not fun!). Each push should be off the center of the blade, but when you're practicing focus not on the center but the back to get it more correct - once you know you are too far back on the blade when you push off you can very easily shift things a little forward - the hardest part is not being too far forward.
Spins are a more delicate matter, and while ideal technique can make about any blade work, it's much harder to adjust. I have the hardest time spinning on my current blades. When I switched from Gold Stars (7' rocker, 11.75" long) to Gold Seals (8' rocker, 11" long) when I got better-fitting boots in the past, my spins and overall skating improved and I really liked the change (Gold Stars have the same rocker profile as Gold Seals I think, just the difference in the primary rocker radius). When I ended up getting Pattern 99-profile Paramounts (8' rocker, 11" length), my spins got terrible. This goes to show that blade length and primary rocker profile aren't everything!
Has anyone changed blade size slightly and seen a noticeable difference in the feel?
I think that a better idea than changing blade size, is to change blade types, keeping an ideal length blade for your boots. And have well-fitting boots without any extra length.
How can you tell where the sweet spot should fall?
It's unfortunately a trial-and-error game where your wallet never wins.