I think the point has been missed. There is a 'preview' window but also other preview windows on the Edit tab. It's old software from the early 90's apparently. For those who don't do fractals RENDERING is involved. The rendering outputs to the Gimp canvass. That's were the fractal comes to life, not in the little preview window.
I included the Edit box as well this time. The outer window edges are expandable on this system but not the preview windows. If Fractal Flame is re-compiled with larger preview windows then I think longer rendering would have to take place in the preview windows. This method of only using a small preview is how I make 3d fractal animations.
BTW. Like a lot of early fractal software it's only 2D. If you really want to make fractals I recommend Mandelbulber 2. For that you need a gamer or graphics workstation because of all the rendering. I'm glad this topic came up because it brings us back to art rendering rather than crypto currency.