In terms of weightings, I think that depends on (1) your level of conviction and (2) the risk/reward equation or more specifically your analysis of the real risks that the company faces and how likely they are to happen, and the damage they might do. For instance, I wouldn't put half my investable capital into a microcap or nanocap company with significant debt and very few customers, even if they seemed to be very undervalued and had a good growth runway ahead of them, because there are significant risks if things don't work out for them as they have planned them to. A company in that sort of situation, especially with debt and reliant on a small number of clients for revenue, is at real risk of cashflow issues at some point if things turn pear-shaped on them, and that could mean raising capital at a significant discount, which would dilute any shareholders who didn't tip in more money to maintain their percentage ownership, or, worst case, they could call in the Administrators and go broke.
Another issue is your investment time horizon in relation to risks. We have been talking about Mineral Resources (MinRes, ticker code: MIN) who recently got down to around $30, and are now back closer to $40/share, and because I have an eighteen month time horizon at this point for my investments, and I will then re-evaluate after re-organising my investments at that point, I didn't want exposure to MIN while they were falling on the back of lower lithium and iron ore prices, despite their very solid mining services (iron ore crushing, loading and hauling) business and asset base that meant MinRes were not going to go broke even though they currently have significant debt (in the billions). If you had a five year time horizon or greater that would be a different story - and I think MIN would be a good pick-up at those levels, and they likely still are, and those who were buying down near $30 have made a quick circa 26% gain (on paper at least) in only about one week - which is a phenomenal return if you annualise it! MIN got down to intra-day lows of $29.51 and $29.84 on the 9th and 10th of this month and they closed today at $37.80, so the bottom was likely down around $30. It looked like they were going lower to me, but it looks like I was wrong about that.
Point is, my decision not to hold them down there (I sold out of MIN in the $50s recently) was about risk management and my low risk tolerance and because I thought there was a GOOD CHANCE they could go lower and that their share price might not recover within 18 months. So time horizons for investments are influenced by multiple factors however the main ones are probably your stage in life and your available investable cash (/capital). For anybody with money that they absolutely would NOT need to touch for a minimum of five years, I agreed that MIN was likely a buy in the $30s (again, probably still are), but I continue to maintain that there is still a significant chance that they COULD go lower within that period.
But they aren't going broke, so they could be a great investment at current levels or below for patient money. Not advice by the way, just thoughts.
I personally have my largest exposure to a company that I believe has the best chance of continuing to pay me above-market fully franked dividends regardless of performance (due to their profit reserve - they are a LIC) and that I also feel has no more downside risk than the ASX200 (or XJO - their benchmark index) - or not significantly greater downside risk, in terms of total shareholder returns (dividends plus capital gains/losses) over the next 18 months.
My second and third largest exposures are to engineering and construction companies that many would consider higher risk, but that I consider low risk because of (a) their net cash and zero debt, (b) their track record in their industry, (c) their history of profitability and rising dividends , (d) their very high insider ownership which means their management are well aligned with ordinary retail shareholders, and (e) tailwinds that I think they have now and will have in the future.
In other words, two of my highest conviction companies are my second and third largest positions.
One of my smallest positions is Cooper Energy, because while I think they are undervalued due to their position as a supplier of South East Coast Australian Natural Gas (they're an oil & gas production company that mostly produces gas) and the tailwinds that I believe they will have in the near-to-mid-term future with rising demand for gas in that part of Australia (Victoria and Eastern NSW, including Sydney), my level of conviction in the investment thesis for COE (Cooper Energy) is not as strong as it is for those other companies. And COE do have debt. And they have had significant rehabilitation obligations in prior years, and may have more in the future. So there is more that can go wrong. So I hold a smaller position.
So my weightings will depend on my level of conviction for each company but also what it is I'm trying to achieve with that position, and over what timeframe. Peter Lynch said "Know what you hold, and why." That's my number one rule.
I may add some more here later (again)...