Bet sizing online of course matters. It matters live too. It is actually the most important part of reading your opponents online. Becasue you cant get many
tells from timing and can not see the others faces/actions, then bet sizing and timing is all you have to go on. Here is how most bet size, Top pair=1/3-pot size bet, depending on the tournament/blind levels. Mid pair = 1/3 - 2/3 pot size bet, again depending on the levels and buy ins ect. Sets are hard to put people on but most bet around 1/3-1/2 pot size to try to sucker some people in or trap with a check raise. It is hard to figure out bet sizing when you are new to poker but the best thing to do is keep an eye on someone and how they have played previous
hands, generally if they bet somewhere near 2/3 w top pair b efore they will do it again and likewise if they bet pot size, they will do it again with a similar hand. Now with draws like flush draws and straight draws you will probably see people betting a little lower than 2/3, something like 1/2 pot size bet to see if they can hit it, this is how you get reads on your opponents and you can put them on certain hands. really good players will change up their bet sizing with certain hands in hopes that other good players are actually watching them and try to deceive them to having something they have or dont have respectively. Bet sizing is actually probably the most important part of poker. This is where all the math starts to come into play. The reason for the bets to be a certain size is becasue you dont want too many people to have pot
odds or implied odds even to call the hand. If you make your bet too low respectable to the size of the pot then you will give people
pot odds to call with less strong hands and give them chances to catch up and win the hand without risking too much of their stack. If you bet too high resepctable to the pot you may be risking too much of your stack respectable to the blinds and hand you are playing and give others a chance for a big come up if they hit a big hand or simply scare them away and never get any value with your good hands. This math becomes very deep and a very huge part of poker. I would say 60-70% of poker comes with Math, and it all starts with sizing your bets compared to the pot and other variables (i.e. how many people are on the table, how many people come to the flop, the size of the blinds, whether its cash or tournament, the size of the blind levels, the position on the table compared to the others in the hand). All of these are variables that need to be taken into consideration, but it will all start with bet sizing and all your reads online will come from others bet sizing. Good luck with everything, i hope i helped a little bit, but there are just too many books to write on this subject.