About how wide and how deep were trenches during this period?

Frontline trenches were usually about seven feet deep and six feet wide.

Not sure about which period you ask, but ...

Explanation:

Frontline trenches were usually about seven feet deep and six feet wide. The front of the trench was known as the parapet. The top two or three feet of the parapet and the parados (the rear side of the trench) would consist of a thick line of sandbags to absorb any bullets or shell fragments.