Let out the side seams if there is seam allowance
Remove the sleeves and add fabric panels
Add gussets under the arms
Insert matching fabric strips down the sides
Replace the back panel with a larger piece of fabric
Add a center-back or center-front panel
Use stretch fabric inserts for extra room
Open and resew darts to create more width
Soak and gently stretch a cotton shirt while damp
Use a shirt extender or layering panel
Buy a larger size and tailor the fit
Take the shirt to a tailor for alterations