What's the diameter of the tube 100 or 110mm?
Extruded acrylic tubing is nearly always 3mm cross section, and the internal tolerance is usually reasonable, but can be very variable- some suppliers are better than others.
To size an o-ring, remember the diameters specified are always plus double the o-ring thickness. For instance a 90 mm o-ring 3mm thick will be 96mm overall diameter, and that coincidently is the size of o-ring that would be ideal for 100mm acrylic tube.
For 110mm you would want a 96mm o-ring if using 3mm cross section. The diameter of the endcap groove should be about 0.6-1mm greater to allow the o-ring to squish inside the tube about 15-20%. The same grovve should also be about the same amount wider, as the rubber extrudes but doesn't compress, so you have to ahve somewhere for the rubber to squeeze into. So for instance the endcap groove for a 100 mm tube would need a groove of about 90.6-91mm diameter and 3.7-4.1mm wide.
Most people use nitrile o-rings, but you can also use silicone o-rings. The latter are softer, and better if the tubing accuracy is a bit wavy. You mustn't use vaseline or silicone grease on silicone o-rings though- it rots them. Lard or vegetable oil works great. Nitrile will take pretty much any mineral or silicone based grease/oil.
One final point, make sure the tubing edge is chamfered with no sharp edges, to avoid slicing the seal.