Simple solution:
Either: Run ANALYSIS/UnitsDimensions. This gives the dimensions of the whole visible scene's bounding box. To scale the scene you need to set one desired size for X, Y or Z in the tool options flyout. There's no option to put in some scaling percentage or factor, so you'll need to calculate the length of one dimension externaly. All others will be calculated along to give proportional scaling. This tool also allows to click one point and drag to another one. If you know the target length of this line (e.g. the length of a leg) you might put it in SetTargetLength and MM scales the total object based on this threshold.
Or if you want to scale a single object (maybe there are severals in a scene): Run EDIT/Transform. Assuming you want to scale it proportional make sure UniformScaling is enabled in the flyout. Now double click on a "Size..." number to put in a target dimension value along this axis or on a "Scale..." number to set a scaling factor (75% means 0.75).
