Over the years, I have grown tomatoes and cucumbers without any trellis or with the classic tomato rings, but letting tomatoes in particular crawl along the ground invites bugs and slugs. Cucumbers also can take up a lot of space and getting at your harvest can be tricky. Both cucumbers and tomatoes can become quite heavy, so make sure the trellis design you pick is heavy duty enough to handle the weight.
I have had tomato plants taller than me before, so I went with a trellis roughly my height. I was tired of the tomato rings bending, breaking, and falling over from the weight of my large tomato plants. This trellis will likely be temporary as we plan to move some of these crops to raised beds in the future.
We had some spare lumber left over from building a shop/greenhouse, so we picked some similar sized boards and cut them to match. We then screwed then together and even attached shelf bracing to the corners to add strength.
We used a post hole digger to easily dig for the trellis legs. It is not 100% straight but was good enough for our liking and will hold up quite a bit of tomatoes and cucumbers. I also plan to grow my Cucamelons up this trellis. I can alternate between cucumbers and tomatoes on whichever side to practice crop rotation.
We made it about six feet tall to accommodate fairly tall tomato plants. Anything taller can grow over the top.
I tied twine at the top down to each tomato plant and tie it loosely to the base of each plant so it can grow thick and be guided up the twine. Same for cucumbers once they are large enough. You can then harvest from both sides of the trellis and the plants get more sunlight and air for a healthier life.