I thought I was done with on prem with only a single SBS 2011 box to support but now I have acquired four new clients all running Exchange 2010. One of these in particular has a fairly large public folder database (the file is around 300GB), I will aim to migrate all of these customers to Office 365 and am fairly confident in managing that for the user mailboxes but I'm unsure of the migration path for these public folders, they are used to store mail items relating to various projects/jobs.
I have been looking at the various materials and I'm still not clear on the best path forward, do I just migrate to modern public folders in Office 365 and the users carry on the same way as they always have or do we migrate to Office 365 groups? Will that size be a problem or is this fairly typical?
Size is not a problem, you can get up to 50TB of Public folders quota in ExO. As to what target to migrate them to, that depends on many factors. Best talk to the relevant owners and try to find out how the PFs in question are being used, so you can plan accordingly. Microsoft offers tools to migrate them to ExO PFs or O365 Groups, but there are third-party products that can migrate them to other targets as well, such as Shared mailboxes.