I think xunilk's answer is the best overall solution, but here a couple of suggestions that could improve your current workflow.
You can use the identify tool to get polygon area. Just select the Identify tool, click on the polygon and expand the "derived" section of identify results. You can even choose the option to "expand new results by default" so you don't have to open the derived section each time. (You can change the area units in Project menu > Properties > General > Measurements.)
The "Split Features on Steroids" plugin allows you to
Split one or more polygon/multipolygon features showing the resulting areas on each side of the cutting line. Adjust the vertices of the cutting line before splitting.
So you could create a too-large polygon, use this plugin to split off a polygon exactly the right size, then delete the unwanted portion.
Note: QGIS plugins are created by outside developers, much like third-party apps for your phone. Each plugin must be updated to maintain compatibility with QGIS. If the plugin developer has abandoned it, then it may never be updated. You can watch the github page for the QAD plugin for updates. Also keep checking the plugin manager for new plugins, in case someone creates a new plugin that does what you want.