The correct one is “I will be a bit late” and you can say “I could be a bit late”! The first one means that you definitely are going to be late and the second one means that there is a possibility that you are going to be late
people normally use "I will be a bit late" it's not really normal when people say "I would be a bit late". normally the term "I could be a bit late" is when you dont know if you will be late or not. if you are positive you will be late, use the term "I will be a bit late"