Help sizing PEX-AL-PEX for hydronic heating

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So I had to do basement renovations which led to removing the majority of my existing hydronic heating line feeds(amongst lots of other things). It was fairly haphazardly run, so this wasn't the worst thing. Previous setup was direct return 1" main with mostly 1/2 feeds to the radiators. There are 6 radiators total, 2 on each floor/3 floors. The total BTU output of the rads is just under 38k.

My plan was to run a reverse return loop, and I am planning to use PEX-AL-PEX. My question is what size the main loops should be - 3/4" or 1". Previously 1" was used, but I think what I'm generally seeing is 3/4 PEX-AL-PEX should carry this? Plan was 3/4 main loops with 1/2 line to the rads off that. Thanks for any insight you can provide.

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