The fabric's count decides the finished size. A one-line sum tells you what you will get, and how much thread to buy.
This is the question that comes straight after "how many stitches wide", and the two are the same question from opposite ends. The chart gives a number of stitches; the fabric decides how much room a stitch takes; multiply them and you have your finished piece, in centimetres, on the table.
The number on the label (11, 14, 16, 18) is stitches per inch. So 14-count aida, the commonest and the easiest to start on, is 14 stitches in 2.54 cm, about 5.5 per centimetre. The bigger the number, the smaller the stitches: finer work, longer work, and better eyesight required.
Divide the number of stitches by the fabric's count, keeping the units the same. In centimetres: stitches ÷ 5.5 for 14-count. A chart of 80 × 100 stitches is therefore about 14.5 × 18 cm on 14-count, 18.5 × 23 cm on 11-count, and 11.3 × 14 cm on 18-count. One chart, three very different pieces: the fabric decides, not the chart.
Add at least 5 cm of fabric on every side, so 10 cm to the width and 10 cm to the height. This is not waste: you need enough to hold in the hoop, enough to frame afterwards, and enough to recover if the motif drifted. Fabric cut close to the motif is a piece that can never be mounted.
Stranded cotton is sold in skeins of six strands, separated before stitching. Two strands on 14-count is the usual choice: good coverage without bulk. On 11-count people often go to three, on 16 or 18 to one. If the fabric shows between your stitches, add a strand; if they look padded, take one away.
One skein of DMC stranded cotton is 8 metres and covers about 1,500 stitches at two strands on 14-count. The legend on charts from here gives the stitch count for every colour: divide by 1,500, round up, and that is your list. In practice most colours in a chart fit in a single skein, and only one or two need a second: the background, and the bulk of the subject.
Ecru and white sell most, and ecru is the more forgiving: pure white shows up every hole left bare. If your motif has a lot of pale background, fabric the colour of that background saves you thousands of stitches: you stitch only the subject and the cloth does the rest. That is also what the tool's background removal does, but earlier.
On 14-count aida, margins included: 50 stitches is about 9 cm of motif and a 19 cm piece of fabric; 70 stitches, 13 cm and 23 cm; 100 stitches, 18 cm and 28 cm; 150 stitches, 27 cm and 37 cm. Past 150 stitches wide, count in months rather than evenings.
It costs nothing, it takes a minute, and the chart is yours.