Telerik ASP.NET RadEditor is adding SPAN and FONT-SIZE tags to my text

I'm including the response from support below in its entirety; but the fix (for me) was this:

StripFormattingOptions="Span, Css, MSWordRemoveAll" tells RadEditor to remove inline <span> tags and CSS styles that browsers inject during editing or pasting. This keeps your stored HTML clean and prevents inconsistent formatting.

This is typically a browser behavior (especially Chromium-based browsers like Chrome and Edge) where the browser inherits or inserts inline styles from the parent page or content area into newly created elements — particularly when deleting and re-typing text. Here are some resources and approaches that may help:

• How to Stop Auto Insertion of CSS Class in RadEditor when Deleting Text — demonstrates how to use OnClientLoad + onkeyup handler to strip unwanted <span> elements inserted by the browser on backspace/delete.
• Style is inserted in text when deleting paragraph tag — covers the same Chrome/Safari behavior and provides solutions for both ContentAreaMode="iframe" and ContentAreaMode="div".
• RadEditor adds span tags around text (forum thread) — discusses how page-level CSS styles can bleed into the editor content area and cause unwanted <span> wrapping.
• ContentFilters property — ensure the OptimizeSpans filter is enabled (it is by default). This filter merges and cleans up redundant <span> elements.
• StripFormattingOptions — if the spans are introduced on paste, you can configure stripping options, e.g.:

• <telerik:RadEditor runat="server" ID="RadEditor1"

    StripFormattingOptions="Span, Css, MSWordRemoveAll" />

Regarding HTML encoding

RadEditor does not HTML-encode content by default. If you're seeing encoded entities (e.g., < instead of <), this is likely happening either during save/load from the database or due to a custom content filter. 
Please also examine the dedicated article about: Appearance of Odd Characters After Saving RadEditor's Content In Database.