The following Sizing Guide is taken from Peter England, & is offered for your reference.
They recommend to enjoy the best fitting from your shirt YOUR Chest & Waist measurments do not exceed these maximums :
Size 18 1/2" Collar - up to Chest 54" & Waist 54"
Size 19 Collar - up to Chest 56" & Waist 56"
Size 20 Collar - up to Chest 60" & Waist 60"
Size 21 Collar - up to Chest 63" & Waist 63"
Size 22 Collar - up to Chest 68" & Waist 68"
Size 23 Collar - up to Chest 70" & Waist 70"
ie: if your chest or waist at the largest exceeds 56" you would need to go up to a 19 1/2" collar even if your actual neck measures only 19", otherwise the shirt will pull & gape when buttoned up or you sit down.
Conversly if you require a 19" collar to be comfortable round your neck you would need to select a 19" even if your chest/ waist is much smaller in proportion than the sizes shown. The shirt body may therefore be slightly looser fitting than you prefer ideally. This could be overcome by sewing a dart in the back. Alternatively we are pleased to offer stock from a range of suppliers, with some makes being less full cut than others which may be preferable to some customers and this is indicated (Fuller cut shirts include Peter England & Double Two, Colin Ross offer a generally Slimmer fit)
As a general rule of thumb most of our customers who take a '3xl' in casual tops wear around a 19" collar / '4xl' 20" and so on