It's not clear to me exactly what you want.
If you want "to export my image from Lightroom in its original size", you can't get b0 size.
On the other hand, if you want b0 size, you must crop your original photo from its present size (5472x3648) to an aspect ratio equivalent to b0 (which is 707x500, or some multiple thereof).
Since you say "Lightroom is making it smaller", that happens when you export with certain settings checked. If you UNcheck "resize to fit" then Lightroom will keep your pixels unchanged at 5472x3648, but as I said in the previous paragraph, that will not print b0 size.
Even when I Uncheck "resize to fit" the photo becomes smaller.
Basically when I open to image on Photoshop it has the original size, however ,if I import it to Lightroom, adjust it and export again to save it, the photo is maximum 50x40cm.Even if I am not resizing when importing or exporting.
What is the maximum size I can have my images when working with Lightroom please?
You need to give specifics, what size does it become? And if you're looking at file size, you are looking at 100% the wrong measure. File size is irrelevant, meaningless, misleading, and you should ignore it.
Now ... what is the end goal of all of this activity? That part still isn't clear to me, as you seem to be describing intermediate steps. Is the end goal to print the photo at b0 size? Yes or no?
Digital files don't have sizes in centimeters or inches, meters or yards. They have only pixels. If LR is not changing the number of pixels, it is not changing the digital image. When the image is printed, the number of pixels packed into every inch has a large effect on the quality of the print. This is the pixels per inch number, or "ppi". For a smooth print paper it is generally accepted that 300 ppi produces optimum quality, but for canvas a lower ppi, as low as 150 even, will do. Unfortunately, your camera puts a tag on the digital file which sets a default ppi of only 72. The reason is very anachronistic and silly and is a hangover from 30 years ago when Apple monitors had a resolution of 72 dpi, a measure long ago abandoned. That tag is misleading because it gives the impression that a very large print can be made from the pixels just as they come from the camera (the smaller the ppi, the larger the print) and some beginners think that is the only size print that can be made. But that not true. It is only a default tag, a suggestion, and it can be disregarded or changed (LR changes it to 240 ppi) and any sized print made. The real, operative, ppi will be the pixel dimensions divided by the print dimensions. Your image with all its pixels (uncropped other than to the 1:1.4 paper ratio) will have an operative 93 ppi on b0 paper which is a bit low. I would suggest using LR's resize function to double it to 7296 pixels on the short side.