The higher the flow rate for a given diameter pipe/tube, the higher will be the resulting hydraulic friction head loss (typically measured in feet). The smaller the pipe/tube diameter for a given flow rate, the higher will be the hydraulic friction head loss. The higher the hydraulic friction head loss, the higher will be the required head supplied by the pump (also typically measured in feet) to achieve the required flow rate. So there really isn’t a maximum flow rate for a given pipe/tube diameter. The problem is selecting a pump that will provide BOTH the required flow rate and required head for the PEX hydronic network.
Typically, you don’t want to exceed a circuit length of 300, 450, 600, and 750 feet for 1/2, 5/8, 3/4 and 1 inch diameter PEX, respectively for typical residential serpentine pattern PEX layouts. You should really consider using hydronic radiant floor heating design software to determine the PEX diameter, PEX spacing, PEX circuit lengths, manifold balance valve settings, circuit flow rates, circuit friction head loss, circuit supply/return temps, and select pump(s) that will provide the required hydronic radiant heat gain (which should first be determined by performing a proper room-by-room heat loss analysis). We have free DIY heat loss analysis and hydronic radiant floor heating design software on our website that you might want exercise after first reading the associated instructions:
Borst Heat Loss Analysis Software Borst Hydronic Radiant Floor Heating Design Software 1/2 inch diameter PEX is likely the most common size used for residential hydronic radiant floor heating applications.
If you want to explore and better understand pipe hydraulic friction, perhaps exercise our pump & pipe design software:
Borst Pump & Pipe Design Software (Hazen-Williams) Borst Pump & Pipe Design Software (Darcy-Weisbach)