The problem noted by the OP has existed for a while in the Fabric editor, and is really annoying. Actually, it was originally true that inline dynamic content macros could only achieve a max width of 300px, but could be sized smaller if the content allowed it. Sometime in the last year or so it seems that the size of inline dynamic content macros became fixed to 300px, regardless of the sizing needs of the content. These constraints make inline dynamic content macros only marginally useful.
We will do some research here shortly to get a sense of whether or not some of these assertions are still true, but we’ve reached out before on this via both CDAC and DEVHELP to no avail.