I could take dimensions on mine to give you as an example of what NOT to use.
You really can figure out your own better than anyone can provide them to you I think.
What is a comfortable seat height for you? For me it's 15" but I'm fairly short at 5'8" for you it might be different. That determines you base line for all other dimensions. You want to shoot a rifle sitting upright not bent over like you are laying down. That is where I went wrong my bench top is way too short for the seat height which I did get right on mine for me. As a result I have several 2x10 boards stacked on top to bring it up to the right height.
Another critical matter is whether the range is flat slowping up or sloping down. All make a difference in the angle the bench sits and the height of the top from seat.
Do you shoot left or right handed? Do you want a bench where both right and left hand shooters can shoot? For both you need a cut out in the middle and I'll never build another without that feature even tho I shoot right handed only. How wide that opening needs to be depends a lot on how wide you are. I'm kinda wide so need a wider opening to be comfortable. Measure the width of a chair and add at least a couple inches.
Front to back my current bench is about 36" and that is to me a very absolute minimum and when I build my next it will be closer to 48" to give more room for things on the bench. I'd call 42" an absolute minimum for real comfort. How wide to make it is again a personal matter. For me the opening to sit down into and thus width of seat should be around 27" and 30" might be better. For you it might be less. Then to each side of that opening at the back you need room for the butt of rifle and sandbags so firgure not less than 12" and not more than 18" to each side that you want to have a side hang over. That pretty much gives you what you need. Adjust it to fit you.
