A 300-nt in-vitro transcribed RNA will be easily detectable on a gel of 10% polyacrylamide (acrylamide:bis-acrylamide=19:1) / 7M Urea / 0.5X or 1X Tris-Borate-EDTA buffer at pH ~8 - 8.4 [see below]. Normally (i.e., "in the old days"), these gels are 1-1.5 mM thick and run at ~50 degrees C (for full denaturation of the RNA -- it won't degrade at 50C). The heat must be distributed uniformly across the horizontal dimension of the gel in order to prevent the RNA band from "smiling". This can be done by having one or both surfaces of the gel surrounded by buffer, or one surface covered with an aluminum plate.

[Aside: There are various formulations of TBE: most common ("1X") is 89-100 mM Tris base, 89-100 mM boric acid, 1.25 mM EDTA. Many recipes have 2.5 mM EDTA, but this causes too much Joule heating in the gel. If you want to run gels fast, you can avoid over-heating by using "0.5X" TBE: 45 mM Tris base, 45 mM boric acid, 1.25 mM EDTA. EDTA is added from an 0.5 M stock solution brought to pH 8 with NaOH.]

Thank you both for your responses - it;s a bit late, but I do appreciate your answers