The lightest cargo is ~5 tons and the crates ~9 tons, but heavy cargo is 20+tons. Most is below 15, so that's a good general point to aim for (in my opinion).
To answer your question based on my experience (so take that with a grain of salt, I may make mistakes and everyone makes ships differently). I'd aim for a sub-5,000 ton ship with 5 fuel chambers and 15 generators, 60-72 t2 box engines (easily divisible by 4 to keep things simple), so 33 million to 40 million thrust (40 million will get you near max speed empty which is a nice convenience).
If it's a medium/heavy miner, I'd consider adding an extra gen - with 6 t2 chambers/18 gens you can just about handle 3 mining lasers with full draw which is nice.