I like this level, especially the layout. The aesthetics and music were also good.
A few things though:
This arrow points down, and blindly trusting coin arrows, I jumped down...
...only to fall into this hole on the right. I suggest moving the hole farther to the right, so there is more room for the player to land. Also the jump to the right can be quite tricky, because you don't always see the koopa and can get hit while you land on the right side.
When you have destroyed all the koopas, you can't hit the yellow switch anymore (at least when you're small, it might be possible when you're big) and get stuck.
In the part with the falling donut blocks, you might want to replace some of them with the slower falling ones, to make it more fair.
And because of the length of the level, an extra mushroom would be nice. Although there is a power up hidden in a secret area, most people (including me, before looking at your level in the editor) would not find it.
But aside from these small things, I enjoyed playing your level and hope to see it finished soon