Use Grid to Make All Letters Same Size

Changing Grid Properties

Create letter B the same size as letter A.

Create a new layer by clicking the tiny arrow next to the new layer icon on the Layer Palette and select 'New Vector Layer'. The 'New Vector Layor' dialog box will pop up...enter B for layer name.

On B layer, we'll draw ellipse in middle of canvas that is about same size as letter A.

You can make your bubble letters very freeform and approximate the size or you can make each letter fit into a specific dimension.

To make letters fit a specific dimension:
Select View>Change Grid, Guide & Snap Properties (Grid tab) or right click image title bar and select Change Grid, Guide & Snap Properties.
In the 'Current Image Settings', enter the size you want. (Leave 'Default' as is.)
For example, if you're making a quilt with 4 inch squares, you might want each letter to be a maximum of 3 inches in any direction. In this case, you would enter 'inches' in the 'Units' box and, for 'Position', enter 3 for both horizontal and vertical.
Now select View>Grid from main menu or right click title bar and select Grid or use Grid icon (if you've placed it on a toolbar). Repeat to turn grid off.
Now you can use the grid as a size guide for each letter. Adjust the A that was already created, if needed.

Hide A layer. Now that we have a size guide, we can hide visibility of A layer. Click the 'eye' icon on Layer Palette to hide layer. As we create each new layer for each new letter, we will hide visibility for previous letter. Keeping previous layer hidden is not only less distracting, it will also keep the tools from accidentally selecting part of the previous letter.