I'm a little late to this particular show, but I'm wondering where is the added value of an additional program that simulates a 6" screen? Isn't the whole point of reflowed text that it resists fixed-dimension formatting? As some else mentioned, there are a slew of other screen sizes available for consuming ebooks, from computer screens to tablets to smart phones. But even if there only existed one screen size for ebooks, every e reader I've seen allows the user to change font-sizes, and most allow the user to change the font-family. Indeed, It is exceedingly rare that I see two consecutive e-readers on the subway using the same display settings.
Now i understand the desire for at least some level of stylistic control (otherwise we'd all be making plain-text books), but you should be able to roughly approximate your ideal dimensions by fiddling with window sizes and text magnification in Sigil itself. But since you can't reliably predict what a potential readers display settings will be, there's really no need (as far as I can tell), to look for an exact replication of settings that you've arbitrarily deemed "standard".