The easiest way to understand the size of viola d'amore is to compare it to a modern viola. The size of a viola is always measured from the back. From the edge adjacent to the neck over the arching, and it does not include the neck button. - For Viola d'amore I measure from where the end of the neck joins the ribs.
Small viola 390mm 15 1/4"
Medium viola 400mm 15 3/4"
Large viola 420mm 16 1/2"
Minimum size for a 7th string
The Chardon and Fusi 7 + 7 string instruments i make with a 420mm back, they have excellent bass and good 7th string bow response and volume. It's possible to build a 7 + 7 at 400mm but the bass string will be weaker. Smaller than 400mm it's not really worth contemplating the 7th string.
Advantages of the larger 7 + 7 string models
You can play a larger repertoire with the extended range. They are louder and have more volume and sustain to the resonance of the symps. Better bass and the tone of the high register of large viola d'amore is quite diferent to smaller instruments.
Advantages of smaller 6 + 6 models
They are easier to play. The strings do not need to be so close to the edges of the fingerboard. Bowing is easier because the strings can be more widely spread on the bridge and there is a little more space for your fingers too. Smaller instruments are not as loud but they are much easier to hold. The tone is brighter and dynamic in the higher register.
7 string instruments will always be a little more cramped because there is a limit to the maximum playable width of a neck.
In short the 7th string is a trade off of ergonomy and playabilty for the extra bass range. Is it worth it? Yes - but only if you actually need the extra string for what you want to play.
If you are sure that you absolutely need 7 playing strings but are worried that you will struggle with a large instrument I'm happy to build at whatever size you think you can manage.