How long should my snowboard be ?

Depends on the type of riding you're looking to do but since you're a beginner most new people are better suited starting with an all-mountain board essentialy a middle of the road board that doesn't excel at anything but is decent at everything "freestyle,freeride,pow ect ect". 154 would be fine, you could have downsized a tad but it's not going to be anything so large that you'll feel overpowered by.