I take 7 1/2 in Clarks and 8 in some others. I got a pair of Lidl MTB shoes size 42 - 8 and a pair of dhb r1's size 43 - 8. Both are good fit. Try them before buying as different manufacturers shoes sizing will be slightly different. Lidl has 42 as size 8 and Wiggle has 43 as size 8 .
Another thing to consider is are they summer or winter shoes? I like to wear big thick woolly socks in winter so a tight fitting shoe is a no-no,and like GP says different makes will be slightly wider fitting also.Really are best to try before you buy.
I think the sizing will probably depend on personal preference, and on whether your feet are strange at all.
My shoes are usually British size 11 and my Shimano RO-76 shoes are EU size 46, which should be identical. The Shimanos are tighter than trainers but I gather they are supposed to be. Even if they aren't, I don't mind, and find them comfortable.
Why not just do as you would do with any other shoe? try them on.
There is no reason why these shoes should be bigger than normal shoes. You could argue that tight is good as your feet won't move, something more likely and less desirable than in day to day shoes.
Each manufacturer has their own version of a size , just like normal shoes. get some that fit. If for winter try with thick sox or sealskins if you wear them. Consider bunging another foot bed in if they are a touch sloppy with thin sox and take it out in the winter.
oh yeah 42 is a Euro size and equates to a English 8. each Euro size is ( as best can be done bearing in mind that they were never meant to be converted) 3/4 of an English size. so 43 is 8 3/4 and 44 a 9 1/2.
Throw in the fact that every manufacturer will have their own sizing and its nowt but a guide.
After searching for a long while foe a decent fit (most were too narrow for me), I can only recommend trying on as others have said. Different brands tended to vary quite a bit. If buying online, then make sure of the returns policy (after finding the Sidi were a good fit for me, I bought online through the Cycle Surgery and when I needed to change to a bigger size they let me exchange them in a shop nr work.