Originally Posted by 11.4
Here's a broader way to think about it:
1. To race cross, you have a couple matters to consider with regards to the top tube. First, you want enough room at the front of the front triangle to stick your shoulder when you pick up the frame. And if the top tube has serious slope, it wants to slide off your shoulder. Second, if you want to do a rapid dismount, you want to have the top tube convenient enough (i.e., right height) so you can grab it with one hand and do a running dismount. When you do that, many/most riders like to just pick up the frame at that point of the top tube and carry the bike -- they've gotten a lot lighter than when you really needed your shoulder to haul them up a hill. Now if you have too much top tube slope, or your frame is so small that your top tube is way below you as you dismount, you aren't going to be able to reach it and dismount effectively.
2. All of this means you don't really want to go to a really small frame or not necessarily one with a lot of top tube slope. This is why Belgian cross frames tend to be big with horizontal top tubes. Or you need to train without the full range of rapid dismount options.
3. If you have a smaller frame size, things like sloping top tubes limit the room for bottle cages, and actually constrains both seat tube and down tube cages. If getting it built for you, have the seat tube bosses installed so the bottle is right down at the bottom of the seat tube (under the down tube cage rather than on top of it). Sure, there are side-entry water bottle cages, but they are just that much easier to pop out when you don't want them to, and whether in cross or in general use, you really want access from either side with either hand.
4. Also in smaller frames, if you get the frame with a 44 mm head tube and a tapered fork steer, remember that you can't make the head tube shorter than the length of the taper, which for Enve is about 8 cm or so. You already have a head tube that's effectively long because of the greater hub to crown distance in a cross fork, so if you want a low position or are on a small frame, you don't necessarily want a particularly tall head tube. This then means you have to raise the seat tube to keep the saddle to bar drop a constant measure. And that in turn means you are raising the bottom bracket.
5. Don't worry about standover height. I hear people still complaining about it, even when they already tilt the bike over a bit to put a foot down.
6. Don't worry about toe overlap at the front wheel. It's the last measurement to give priority to when planning a custom geometry. More bad bikes are designed around toe clearance and in truth, I have a good 3 cm overlap on all of my bikes and I occasionally brush the front tire as I dismount but not while riding.
In short, recommendations to downsize frames are irrelevant. Depending on your frame size, you may have some extraneous constraints, but generally, fitters and even builders often don't take into consideration issues like reaching for your top tube for a running dismount, or getting a low enough position on a small frame without running out of head tube, or using a sloping top tube to lower standover height but losing a frame that shoulders well. You basically want to shoulder the bike or you want to carry it with a hand in the middle of the top tube. Shouldering is the traditional way, but it evolved for bikes that were really heavy and somewhat bigger than we need today (thanks to nothreadset headsets in large part). If you watch the fastest riders, they tend to do a running dismount. If you can do this you want to pay attention to having the top tube high enough so you are grasping it as you swing off the saddle. Everyone does this a little differently, but it comes down to a need in designing a cross frame to consider exactly how you'll race and dismount. If you're doing a gravel frame, I really like to be able to do a rapid dismount because it's an escape route if you start to slide in the gravel or lose traction on a steep uphill. And on a gravel frame I want enough room inside the main triangle to accommodate water bottles plus the option of a small frame bag. Add a frame pump if so desired on the gravel bike and that's even more room needed inside the triangle. Even on a big frame, it goes pretty fast, and you have to remember that that tall cross fork basically reduces your main triangle size pretty rapidly. If you want a nice low bottom bracket on your gravel bike, you are shrinking the triangle (and your head tube) even further.
So anyway, these are just a few observations that might help your geometry planning. It's always fun to be told what you can't do, isn't it?