Newby to MTB, frame size for me?

[Frimley111R]

Very basic question here but I am dippin' my toe in the MTB water and am going to by a 2nd hand bike from eBay. I don't want to waste £500 if I don't use it. However I am not sure, at 5'8" tall, what size frame I should go for. I went into Halfords and sat on one with an 18" frame which seemed fine but there is a nice Carerra Vulcan on eBay with a 16" frame. Would it be a bit small or is it largely irrelevant given how much the seat moves?

[Frimley111R]

Thanks all!