How much text actually fits
Personalised balloons are printed or cut in vinyl and applied to a curved surface, so the usable area is smaller than the balloon looks. These are the conservative end of what personalisation shops quote — under these counts you are fine basically anywhere, and over them is a reason to ask the shop rather than a flat no.
11" latex
20 characters · 12 per line, up to 2 lines · 284 messages fit
11" across · print area about 5 × 1.8 in
Curved surface and a small flat print area. One short line reads best; two is the ceiling.
18" foil round
32 characters · 14 per line, up to 3 lines · 598 messages fit
18" across · print area about 9 × 4.1 in
Usually already printed with something. Personalised text shares the face with whatever design is on it.
16" bubble
26 characters · 12 per line, up to 3 lines · 503 messages fit
16" across · print area about 7 × 3.7 in
The small clear stretch bubble. Vinyl goes on the front curve, so keep lines short and centred.
20" bubble
45 characters · 16 per line, up to 4 lines · 645 messages fit
20" across · print area about 9.5 × 5 in
The common personalised gift bubble. Comfortably fits a name plus a line.
24" bubble
65 characters · 19 per line, up to 5 lines · 646 messages fit
24" across · print area about 12 × 6.7 in
The big one. A full sentence works here, and it is the only size where a long message stays legible from across a room.
letter balloon banner
14 characters · 14 per line, up to 1 line · 88 messages fit
Each balloon 16" tall, about 13" of wall apiece
One balloon per letter, so the message length is a shopping list. Spaces cost you a gap, not a balloon.
Three things that catch people out
Spaces count. Vinyl is cut as one piece per line, and a space takes the same room as a narrow letter. “Happy Birthday Mom” is eighteen characters, not sixteen.
Line breaks are yours to choose. Most shops apply the text exactly as you type it, including where you press return. If you do not specify, you get one long line in a smaller font.
The balloon size is real; the print area is an estimate. An 11" latex is eleven inches across because that is what it is sold as. The print areas here are worked back from the character limits, so they are a guide to proportion — right enough that a preview shows you truthfully how much of the balloon your words take up, not precise enough to hand to a printer.
The widest line is its own limit. A balloon is round, so only the line through the middle gets the full width. A message can sit comfortably under the total and still have one word too long to set — which is why every size above lists a per-line count as well as a total, and why the previews wrap the text rather than just counting it.
Letter banners are a shopping list. One balloon per letter means a fourteen-character message is fourteen balloons plus the spacers, which is a different order of cost from one printed bubble.