cucumber seedlings - how big before pre-emergent?

Hello.... I am usually on other forums here at DG... but have a seedling question.

I have a row of cucumber seedlings (Sweet Success and Summer Dance) up that are - oh - about 6 to 8 inches high. One variety has a good slze "true" leaf, and one smaller leaf... but the other variety only has one true leaf so far.

I have an injury in my shoulders and weeding is painful - so this year I want to use pre-emergent around my few cucumbers and tomato plants. I plan on putting down newpapers in the aisles, and preen around the plants where the papers won't fit.... then putting a good layer of mulch on the plants. Hoping this will make a LOT less weeding for me.

The tomatoes are all about 2 feet so I don't think they will be harmed. But the cucumbers are not very large yet. My pre-emergent experience has been pretty much limited to the perennial beds or around trees.

How big should the seedlings be before I can apply Preen around?

Thanks!!

Jules