Decide On Percentage Widths In Sizes

[JamieB-gu]

Should consumers avoid using percentages in sizes, or do we allow them to but figure out some way to handle that? The downside I can see to the former is that perhaps it's valid to use a percentage for width - it's a consequence of our implementation that this will also affect the height, not the consumer's fault.

Either way, if we decide to prevent the consumer using them we could use the changes discussed in #7. That way we can restrict percentages at compile-time.

For more context see this discussion and the corresponding PR description.