That said the web offers such great techniques to maintain this. Passive loading of plugins for example could keep things snappy and light and load things when you need them.

If you want the perspective of a prospective long term user: I'd be very comfortable using your app even at tens of megabytes. You could probably keep your initial load pretty light but pull in larger modules as needed. There are certain effects and audio layering I often use in Audacity that would keep me there, but your modern interface and browser access are huge selling points. If your vision includes moving to a bigger editor I guarantee you you'll find a huge base who wouldn't even notice megabytes of code.