7" is fine on a DSM - could be a couple inches bigger but you will do ok. I bet there are plenty of Board members here who do fine. A king or queen size quilt is a bit of a problem but there are a lot of ways to adjust.
Once you get up to speed with FMQ, if you like it, and you want to go gaga, then go for a big harp or a mid or long-arm.
The way I look at it, why spend thousands on something you may not end up liking or being delft at or even wanting to do?