Print it and use a ruler. Compare the relative ratios to the rooms you do know the sizes of.

I am SO not a math person but my husband taught me to use an engineers triangle ruler - believe me when I say if I can do it ANYONE can. Print it out as kirkhall said and work backwards from what you know.

Here is a link that might be useful: Engineer Triangle Ruler

I use AutoCAD 2014 for that.
I can insert an image or attach a PDF but they will be just images that I can enlarge or reduce to scale if I have at least one dimension. I am not able to move the lines but I am able to trace over the image and change whatever I want.
Even without having dimensions, I can scale it pretty close if things like a 5' shower, a 3' entry door or kitchen cabinets are shown on the plan, but it will not be a 100% accurate, in addition, some images are a bit distorted.

A JPG photo image will not retain a scale but often a PDF will. If that is the case, you would use an "architect's scale" to measure room dimensions on a floor plan. An engineer's scale would be used for a site plan.

For a JPG image it is usually easier and just as accurate to print it and make a scale by marking a front door opening width or tub length repeatedly on the edge of a piece of paper. You can fold the paper between marks to get the half distance. This won't work well on screen because computer monitors usually have different vertical and horizontal resolutions.

It is possible to rescale an image in most CAD programs but they are expensive (AutoCAD costs over $4,000) so SketchUp might be a better choice. SketchUp is free but I believe it is necessary to by SketchUp Pro to be able to import files and that costs $500.

PDF XChange Viewer has a free trial version that can do this. You need to know the scale and then you can use the measuring tools to get the dimensions of other things.

You would need your plans in PDF format of course.

You would also need a Windows operating system.