Maybe someone with a similar setup to yours will be able to help you ...
I spent
numerous hours reading through all the online threads about sumps until I decided on what I would build
For me, it ulitmately came down to space and my idea of what I wanted for equipment
You have to do some planning with what equipment you want, and maybe what equipment you may want later on, and make you mind up about how big your sump 'could' be - and if it's under your tank, you have to plan on what it 'has to be'
Read, read some more, and then read more again
We've all been here, and we can't answer these questions for you and you'll ultimately have to make your own choice
As I said, maybe someone with a similar setup can help you, but it's ultimately your decision and we can't give you the ideal choice
You'll have to make that decision on your own
Good luck !!