One point not mentioned is using cheaper or free commission buying. Also, as I'm usually investing for the long-term and holding the share for dividends, I'm less focused on the selling costs.
For my Stocks & Shares ISA I used iii (there are other ISA providers!) which offers a once-a-month day with £1.50 commission, not the usual £10 (it's still £10 to sell). So on £500 that's £4.00 (£1.50 comm + £2.50 stamp duty) or 0.80% to buy. At £1,000 this goes to £6.50 (£1.50 comm + £5.00 stamp duty) or 0.65%. Above £1,000 there's not much further gain in the total costs - a £10,000 block would be £51.50 (£1.50 comm + £50.00 stamp duty) so 0.52% - i.e. you reduce costs 0.15% moving from £500 to £1000 purchase blocks, but only a furher 0.13% going from £1,000 up to a potentially whopping £10,000.
But, the cheaper £1.50 is limited to buying on 23rd of each month - which might not be an optimal day to buy - although otherwise I'm tempted to time the market. Otherwise, at the £10 commission I'm at 2.50%, 1.50% and 0.60% respectively at £500, £1,000 and £10,000 - still more gained going from £500 to £1,000, than £1,000 to £10.000.
So if I'm looking to buy £1000 of a stock I have to consider do I wait until 23rd of the month and buy at 0.65%, or buy in real time at 1.50%. My consideration is do I believe the price might be down by the differential of 0.85% - perhaps some Euro-wobble affecting prices in the short-term - that means I'll buy today for the full £10 commission, in the expectation the share price will increase by at least 0.85% to justify not waiting for the next cheaper buying day on 23rd.
I played around in a spreadsheet looking at ratios of costs to investment blocks. I'd recommend you doing the same to understand where you're comfortable investing. For long-term buy & hold (LTBH) I'm at £500 if I can buy on a cheap day, or more generally £1,000, but that's also reflective that I'm trying to build an ISA portfolio of 15 to 20 companies, so £1,000 each will build up nicely over a couple of years. At £2,000 per company it would take twice as long to get a diversified portfolio together. Later on I can go back around my portfolio and add further £1,000s to 'double up' holdings, at not much more commission costs.
I dabbled in trying to trade on a FTSE100 ETF (ETFs are commission only, with no stamp duty, so slightly cheaper again). I learned here that to buy and sell quickly, made no sense at less than blocks of £2000 (where £10 in, £10 out, no stamp, meant 1.00% costs - a 3.00% price rise would give me a 2.00% net return). Trying at £1000 has 2.00% costs - and that extra 1.00% erodes too much of the potential trading gains to be had. At blocks of £4000 that £20 drops to 0.50% but doesn't improve more at bigger blocks.
Hope this helps new investors. I'm not a total rookie, but have been seriously looking at investments - and investing - over the last 18 months, so I've been learning by doing.