3D Hubs has partnered with Manufacturers who own machines in a wide variety of build sizes. Different locations might have different capabilities available and the system will notify you if your file is too large.
Generally, this breaks down as follows:
FDM - should not exceed 50 cm x 50 cm x 50cm
SLA/DLP - should not exceed 60cm x 60cm x 60cm
SLS/MJF - should not exceed 60cm x 30cm x 50cm
CNC - 100 cm
For files that exceed talk to the sales engineers