I am really new to Tenkara but so far have only found two unsuitable locations for my 11' Iwana. The first location was a really small stream with a TON of dense rhododendron that I fished this past Thursday. I passed on Tenkara and instead fished that stream with a 5' regular rod and even then had to skip over many spots. Could I have used a Tenkara rod, yea probably but it would have been as much fun as a 9' regular fly rod; it would have worked in spots but moving around would have been a hassle.
The second stream is a very famous trout stream that is difficult to fish everywhere but the particular section I was fishing has no way to wade it at all or get really close to fish or suspected lies without exposing yourself and spooking lots of fish. What is needed here is a cast of at least 30-40 feet made from way downstream of the fish. I tried my Tenkara rod and did catch a few fish, but not as many as I saw darting away!!
But what I have also discovered is how many places are PERFECT for the method and tackle, even small streams that you would suspect would give you trouble. So far I get way less overhead snags than I would in many places because I have a fixed amount of line to manage. When I use a regular rod on some of these places when I need distance I tend to stay put and lengthen my casts. That's when I get snagged. With my Tenkara rod, if I can't get the distance, I move closer. Of course moving is an option with any fly rod but most of us don't and instead lengthen our casts which can be a disaster on smaller streams.
The bottom line is, you won't know until you try.
And don't sweat the fish size thing either!