230 litres sounds about right for the loft header-tank, as it feeds the cold input to the shower, as well as repleneshing the indirect hot-cylinder.
I have used a power-shower, supplied from a 210 litre indirect cylinder and loft header you have recommended to you.
After a year of 3 showers back-to-back, the twin-impeller pump siezed as it was not continuously rated.
Choose your brand and power of pump carefully ;--- durability costs money.
ps My loft header-tank has a 22 mm feed to the ball-cock.
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