Is it only me but it seem like there is higher rejection rate if batch is big. Sometimes I feel that 5 a day is most optimal size.

As a volume submitter, I think I'm qualified enough to debunk this myth:

NO, big batches do not create more rejections. What does, are large batches with similar images, or just plain old bad work.

I submit once a month, and my bathes are anywhere from the mid 200's to the low 300's and I only ever have about 5% rejected, at most maybe 8%.