Hi KangNi, good to see somebody here besides me, haha.. I use the spectrum on the bottom of the Right Sidebar, under Track Information, and find that I can make its window any height I want by just dragging the top bar (Visualiser Header Menu bar) in steps.
However, when I moved it to the Bottom Panel just now, I found the same thing you did--a fixed height and no bar to drag. I also noticed that it's missing the "Visualiser" header menu--normally that IS the top bar which is draggable, and what you would expect to see where you want a visualiser element to appear. So, it MAY be that for some reason MusicBee is not correctly including a Visualiser Header Menu (bar) if the element is placed in the Bottom Panel--which might effect ANY visualiser placed there.
Curiousity may have killed the cat, but I felt lucky and tried choosing a "stock" visualiser instead, "Interference", and placed that one in the Bottom Panel above Player controls and status bar, and guess what---no header bar, and same fixed height! So, it's NOT a fault of the Classic Spectrum Analyzer.
I would consider it therefore a reportable "bug"--when a graphic visualiser element is placed in the Bottom Panel, it ends up at a fixed height with no Visualiser Header Menu bar. I will report same and we'll see what happens.