I've owned two belt sanders, both 4x36". They are not the biggest or strongest, but I don't have the space for anything bigger.
My first one was a ryobi or rigid (i forget) from lowes. It was like $100 or so. I thought it was the greates thing ever. I used mainly the rounded end of the belt to tiller the limbs, until one day all that sanding pressing on the end of the belt, blew up the roller bearings inside the idler cylinder the belt runs over. I took it apart, but it was hopeless ... returned that right to lowes ... that was about 1.5yr of service.
I bought the porter-cable one from lowes because they would only give me store credit, and i didnt need $100 in clamps. The porter cable is a whole different animal. Far more power, much better construction, bigger disc (still never use it. It was under $200 but is a much better machine.
I'm sure a bigger badder machine is even bigger and badder, but if you are on a tight budget, a more heavy duty 4x36 sander can be quite capable.