Ive got some plants like my cucumbers that were started at the same time but one plant is still very small and another is 5 times bigger. Is it safe to say the smaller plant is a dud and just scrap it or will it eventually catch up. Just didnt know if they will grow at different rates

Yes, it's probably a dud: that's why you should always "overseed". With Cucurbitaceae (like cucumbers) I plant 2 or 3 seeds per pot and just keep the best ones. This year I've only planted a quarter of the melons I've sown so far. I have another late variety coming up now but judging by what I see right now no morethan a quarter of them will be fit for transplanting.