Leopold tenkeyless is 14.1 inches. That'd leave you with 7.9 inches. Might be big enough to mouse on, can't really say since that'll be up to your mouse sensitivity. Just to give some context, if you wanna look at my gallery, 2nd picture shows the Leopold on my tray which is 24.5 inches, so 2.5 inches bigger than yours (that's me counting the metal side struts to the width). You can also see what I mean when I say with my larger keyboards, like the Model M, I have like 2 inches of clearance for my mouse lol.
Don't have a Filco though I eventually want to get one for my browns funnily enough, but I can say that the Leopold is a fine keyboard and probably very very close to the Filco in terms of quality. This is going off of people's testimonies who have both. Biggest thing seems to be Filco has a more consistent feeling to their stabilizer keys but for me it's not that noticeable but your mileage may vary. Also on the flipside the Leopold has a detachable USB cord instead of a bolted on one which I rather have, just in case the cord breaks, all I have to do is replace the cheapo usb cable. So pros and cons to each.