If your web gallery only displays pictures at around 'postcard size' (and you've optimised them for web display), they needn't be bigger than about 0.1Mb each. However if your gallery allows users to click on those images (or on thumbnails of them) to access the original images for downloading and printing at high resolution, then (if you've used a digital camera to create those images) each file might be be 5Mb or more. Since one option use files which are 50 times as big as the other, it's impossible to give you a figure for the size of your website.
However I assume that you're creating it, with some suitable software, on your PC (rather than using a system which lets you actually create a site online). If so, that software will almost certainly have placed all of the files, which you'll need to upload, into a single folder. Simply right-click on that folder and select 'Properties' to see the total size of all the files it contains.
Chris