In the past couple of years, it's become easier to find printing services for custom mousepads larger than 3x3 feet. However most printers don't seem to offer a standard size between 3 and 4 feet wide, so a 4x4 foot play area might be a good one to think about because that's a standard size offered by both Inked Gaming (for US customers) and DeepCut Studio (Europe). And in that case, I'd probably try to set the trench diagonally.