Three straightforward pixel layers of 5400 x1957 = 95110200 bytes for 8 bit and 190,220,400 bytes for 16 bit.
However there is something which could make the document much bigger.
Is one or more of your layers a smart object?
Whilst a smart object appears to be a certain pixel size, within that smart object container can be another single or multi-layered document with a much bigger size.
thanks for your reply- Indeed the images are larger than the dimensions of the psd. But even if I extend the canvas-size to the image size i won't get a file larger than 266 MB as *.psb-file. It is an RGB file with 8 Bit color depth.There are no smart-layers but one masked layer.
I can't equate the dimensions you gave in the first post with those in your screenshot (assuming the rulers are set to pixels?).
A couple of things to think about though :
1. For an 8bit RGB image - File size is roughly pixel height x pixel width x 3 channels (4 channels if there is a layer mask) x layers. Divide the answer by 1048576 to get MB
2. With maximise compatibility switched on Photoshop also saves a flattened version of the layered file within the PSD
If you are reluctant to use psb, you can save up to 4GB as a layered TIFF