Last things first - you can't change a line's weight other than the thickness of all lines in your 'style' or setting a 'profile' thicker than the rest - BUT there are third-party scripts to mimic line 'weights' and related properties - e.g. 2D Tools' Line-Style.
If a line is 'free-standing' you can change its length in the 'Entity Info' dialog by typing in a new length - it will extend from its end [i.e. the second point picked when making it, the first-point - the start-point - remains static].
If you make sure everything is unselected [Ctrl+T ?] you can the activate the Move tool and then select any line's vertex [start or end] and move it to a new point - if you know the increase in line length type this in, or if you have some other geometry or guides to snap to you can those.
When in Move if you pick away from the line's vertices on the line itself you them move the whole line unchanged in its length.
If you move a line's vertex and it forms a perimeter to a face [that's not triangular] you are constrained to the plane of the face [i.e. the coplanar edges] - unless you press 'Alt', in which case you can move the line's vertex in 3D and the face with 'auto-fold' adding 'creases' to itself so that faces will still exist for the rearranged [now non-coplanar] set of edges...
If you are uncertain of a line's length and want to set it to something specific then use guides to make a 'framework' to snap to or the tapemeasure tool to get it's length so you know what to adjust it to.
If you have a shape and want to make several edges a certain length - for example a 2D face or a 3D cube the if you select the edge[s] [or for a cube select 'faces'] that are attached to the lines in question and then use Move entering [or snapping to] required amount then all of the lines will adjust in one go.
Hope this helps...