Shorter guns and lower diameter bands are generally easier, often much easier, to load. Single bands are quicker to load than 2 or 3 bands.
Around 70cm-110cm will probably work for you. 75-100cm perhaps better. If I was getting just one speargun, I might get an 80cm-ish length speargun. For you, I would suggest either 1*16mm or 2*14mm - be kind to yourself!
By all means look at future upgrade possibilities though.
E.g.
My first speargun was a Rob Allen Sparid with a single, powerful 16mm bulk rubber-band, which could be upgraded to a bigger band or two band ( double muzzle and spear had 2 notches notches). I tried a single 20mm band for some time but went back to single bulk rubber 16mm eventually - quite enough for the UK fish I encountered
and considerably easier and less bruising to load.
I now have several other spearguns. My favorites are old carbon Omer XXVs 75 & 90cm for UK waters (sea). I generally used 18mm stock screw-in bands they came with, then converted them to 19mm/ 3/4" bulk rubber. But in recent years I have been gradually moving back to bulk single 16mm bands.
More recently I bought a couple of Apnea spearguns to try out, An ST 60 or 65? (too short for general use
). 2x14mm bands (to make up for shortness). The other one is a 90/95cm rollergun. Not sure what rubber that has - but I wouldn't recommend a roller to start with. Both Apnea spearguns have reverse trigger mechd, which adds about 5cm of effective barrel length. But the line releases seem a bit too small to me, compared to my Omers. Slows down loading a bit.