You can divide the target size by the original size to get the scale for the size you want, but it's a bit of a pain having to store the original size somewhere for this
If you run this twice using the current size, it will toggle between original size and scaled size, you might be able to use that to avoid the need of storing the original size...
Wouldn't work well if you want to smoothly increase it to a size (but then you can just use scale ?)
But yeah, would be nice to have an action to resize directly, if you don't want to do the math tho, there's Object Resizer object, but that's Windows only.