I am a bit late kicking in here but first I must apologize to the folks who have bought quality round dehydrators with temp control and fans. I was once fooled by Mr. Ron Popeil and went directly to the Excalibur brand and have not looked back. There are a number of great dehydrators out there which are not square!
As for potatoes, dice them to one half inch. That has been my best experience.
For tomatoes I usually look for the fleshier varieties to dehydrate; Roma, early girl and some cherry tomatoes are great for dehydrating right to powder. As for the beefsteak and big boy and better boy varieties they carry less flesh and more fluid.
If I wish to dehydrate tomato slices I slice an early girl variety. The slice tends to keep its shape better.
Debi, I am pleased that you enjoyed your Vermont experience. In about two years or so when I have my farm in full swing you and Jessie are welcome to visit here. And that goes for all the SMF Clan!