The Wilton wedding chart is for slices that are 1x2x4 (4 is the height of each tier, hopefully
). Those are pretty small slices for the US. I've heard that smaller servings are typical in other countries. I definitely wouldn't halve the Wilton servings...the bride will think you're trying to rip her off, and the person who cuts the cake won't be able to cut them that small without the cake wanting to crumble.
If the bride has invited 250 people, then not all of them will show up, so she may reduce the number of servings she needs as it gets closer to the wedding.
One thing you could do if you're not comfortable making a cake that big is to suggest a main cake that's a manageable size (maybe 3 tiers, 14" 10" and 6"...still big but not massive) and then kitchen cakes to make up the rest of the servings. But a lot of brides want a cake that's really huge and massive-looking and in the end it has to be what the bride wants.