Im re doing my roof and as such my sewer vents at the same time my bathroom is 30 feet form my kitchen should I run a second sewer vent line though the roof for the kitchen or can I use the one for the bathroom

Thanks

Yes! You will need to run a separate vent for the kitchen. Good luck, Tom

Thanks roughly how bug of diamater should it be

For a kitchen sink vent 2 inch pipe is plenty.
If your code allows you may install a AAV. Good luck, Tom