Do you have a pic of the manifold you want to use? Most of the ones for resi use are 1" header with 1/2" spigots for the pex. Commercial ones which can be up to 30 loops, can have 1.25" headers and 3/4" spigots, but they are usually special order.
The pressures and flow rates are lower than for DHW but things leak, non the less so make it as tight as you can.
Connecting copper is dependent on the flow rate needed. 10,000btu is only 1 gpm so, if it is a short distance, you could get away with 1/2" but 3/4" is much more common. If less than 10Mbtu, 1/2" is easy.
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