Unfortunately, not all seeds are guaranteed to germinate... just the way of the world... so let's get on to your second question about pot size!
You've not said whether you're growing photoperiods or autoflowers and that actually makes a difference. Photoperiods can be started in anything, really - lots of people use Solo cups! They can be started in any size container because they can take being transplanted when necessary. Autos, on the other hand, absolutely hate being transplanted and so should be planted in the container they will grow for their entire life in... I prefer to use 3 gallon fabric pots (always fabric! Helps the roots get oxygen and helps the soil dry out between waterings). I've grown a few in 5 gallon fabric pots but haven't really noticed any difference in the size of the plant or the harvest... but there IS a big difference in a) the amount of nutes I have to go through and b) the number of gallons of water needed for their flush at the end of their life... so my choice is 3 gallon fabric pots!
Good luck with your grow and have fun!