This question doesn't seem to be asking the length of the diameter of a circle, but asking how to draw the diameter, so that the following is a solution for the construction.
To solve this problem, use the fact that the perpendicular bisector of any chord on a circle passes thru the center of the circle, and the converse is true as well.
1. Draw a horizontal* line that intersects the given circle at 2 points, A and B. Segment AB is a chord of the circle.
2. Keeping the compass width, approx 3/4 of AB, draw 2 arcs, one from A and the other from B. They intersect each other at 2 points, C and D, above and below chord AB
3. Draw a line passing thru 2 intersections, C and D. Line CD is the perpendicular bisector of AB.
4. Perpendicular bisector CD intersects the circle at 2 points, E and F above and below AB. Segment EF is a diameter of the given circle. ** ***
* Any slanted line woud be OK. The horizontal is just for the convenience.
** This answer may be shortened into 2 steps: 1. drawing a chord, 2. drawing the perpendicular bisector.
***For the center of the circle, just bisect the diameter, or repeat the same procedure on the other chord.