How to group numeric attribute ranges similar to prices in layered navigation?

I've just finished setting up layered navigation for products with different suggested age ranges. I ended up making a new attribute for the layered navigation, with a set of predetermined values. I was also struggling to get fre-form values grouped in the layered nav.

So in your case, you would keep the actual size field and put your product sizes in there for display on the product pages, then you have another attribute called Size Range. In Size Range, you have all of the ranges that you want to be shown in the layered nav, like 145cm-150cm, 150c-155cm, ect. Maybe multi-selectable so that a product that's 150cm could be in both? (or just avoid any overlaps) All of those values will show up in the layered nav, and it will work.

The downside...you're entering the size twice, once for display and once for layered nav purposes, yuck. I load new products with a script using the api, so for me this was a bit of coding and won't require ongoing double data entry. If you're loading a lot of products through the magento interface this might be annoying.