If you ever have a problem with your PST, you can run into some real trouble fixing it if you have a big file to check.
Its much better to have multiple smaller PST's than one large one.
Better yet, migrate all your email into Office 365 and do away with PST's for good.
OSTs - We recommend keeping 3 months of email in the cache to improve the performance of the computer. The default is 12 months of email. It can be helpful to delete your OST every now and again to improve performance (do not delete PSTs!).