to further improve on what toilet here said…
worldmachine has a limit of 32x32 in ue4, you cant import any bigger, but your landscape in ue4 can be alot bigger if you so want, it just wont look good to have worldmachine and hand sculpted next to eachother unless you spend alot of time
What you can do and its what i suggest, decide on the size of your landscape before hand, this can be as big as you want as UE4 can go infinite with its level streaming but keep it realistic dont do as me and go for 16x bigger, id say max 5x bigger and even thats gonna set you back thousands of hours
Also to correct some of what toilet said, you can have several landscapes, I have 5 for my map but they are maxed out in size that can be cooked, I should have sticked with 32x32.
If you plan on making it bigger then 32x32 then make 1 landscape and make it as big as you want, create a new umap file and divide it up so you have equal sizes in both umaps, to move chunks of your landscape into another umap creating a landscape proxy is simple, go to sculpting, manage and use the move tool, this way you can use worldmachine on your entire landscape. Do note its easier to hide the seams underwater, hence why we mostly use islands