As an A* (predicted) A-Level Economics student, whose average score on 25 markers is 21/25 I would recommend first determining what your PERSONAL opinion actually is. For the sake of example, let me say that I believe that economies of scale are not the main factors influencing the size of firms. I believe that these factors are: the availability of labor and the nature of business. Then adhere to the following structure:
1. ALWAYS DRAW A DIAGRAM
2. KAA 1 (Knowledge, Application, Analysis) - This will be your first body paragraph dedicated to the first main influence on the size of a firm you have selected (Availability of labour). I would recommend you abstain from writing an introduction, as it is extremely time-consuming and will most likely not give you any extra points. However, you can use your first paragraph as a sort of a mini-intro by briefly introducing/defining your key terms/concepts (e.g. define economies of scale in your case and/or briefly list other influences on the size of firms). Subsequently, you should develop a cohesive "chain-like" argument (this will be your analysis mark), mention some data, names of firms, and points from the diagram (your application mark), and define + explain some key terms/concepts (your knowledge mark). End your paragraph with a sentence that would reference the initial question (e.g. "therefore, I believe that factor X is more influential on the size of firms over time rather than economies of scale")
3. E1 (Evaluation/counterargument) - Now write a paragraph of a similar structure on the reason why the factor you have discussed in KAA1 is NOT as influential/not influential as you said previously, and why in SOME CASES economies of scale can be MORE/LESS INFLUENTIAL than the availability of labour. I would recommend you be significantly less explicit while writing your evaluation, as it is worth fewer marks than your KAAs.
4. KAA2 - same structure as KAA1, but for another factor that you believe is influential (nature of business)
5. E2 - Same as E1, but about THIS factor
6. Conclusion - briefly summarise your key points in 2-3 sentences, ending with "I believe that the main influence on the size of a firm over time are Availability of Labour and the Nature of Business rather than economies of scale". TIP: Avoid starting your conclusion with "in conclusion".
Hope that was helpful. Good luck!